This guide will try to be as spoiler free as possible (and I will use the spoiler tag excessively to enforce this for those cases where I can't convey the needed steps without being too cryptic about it :)).

Further, in this guide I will focus on only describing the minimum amount of steps necessary to progress. If you follow my guide to the letter you will not get the whole story and you will lose out on some exploration fun.
Thus, I would suggest to actively talk to everyone with everyone about everything on your own. This adds a lot to the atmosphere and provides quite some nice insights into the story and backgrounds of all characters. Also actively go around, explore the ship and try to interact with stuff.

Generally, you can get all achievements (except for the endings) in one play-through. The important decision points where the story branches into differing endings happens way late into the game. I will mark that point in this guide by actively asking you to save and copy the save game :)

• Talk to everyone with everyone about everything. As long as there is a speech bubble above a character, you have not talked to said character about all topics with the currently possessed one.
  
• Do not forget to talk to everyone as the cat and have everyone talk to the cat. There is some character insight to be gained here as well :)
  
• Throughout the game you can always return to Paulo to get some tips/hints on what to do next.
  
• All achievements (except for the endings) can be obtained in one play-through. I will point out where exactly you should save and copy your save game to easily get all endings later in this guide.
  
• Save regularly; while very seldom, weird glitches can happen from time to time.
  
• When a character gets stuck somewhere (happened twice during my play-through), you might still be able to salvage the situation by taking control of said stuck character (or the character blocking the way) and move him/her a bit. If the stuck character is one you have no control over yet, you unfortunately need to revert to an earlier save :(

To save me some trouble in describing the cast during the walkthrough, here is a brief who's who on the ships occupants:
Ferdinand
First mate


Marcurio
The ship's cook


Xu
The ship's waitress


Alton
The ship's DJ


Darryl
A passenger



Donovan
A passenger


Garv
A passenger


Gwen
A passenger


Paulo
A passenger


Quella
A passenger


Sunshine
A cat, also a passenger

???
You, a ghost



To continue making some things easier for me, I will also briefly go over the locations I will later refer to in my guide using these maps.
Lower Deck / B2

1 = Donovan's room
2 = Garv's room
3 = Bathroom
4 = Empty room
5 = Gwen's room
6 = Paulo's room
7 = Empty room
8 = Stairs to middle deck
9 = Quella's room
10 = Darryl's room

Middle Deck / B1

1 = Stairs to upper deck
2 = Marcurio's room
3 = Xu's room
4 = Captain's room
5 = Anton's room
6 = Broom Closet
7 = Ferdinand's room
8 = Stairs to lower deck

Upper Deck

1 = Stairs to middle deck
2 = Stairs to bridge
3 = Dining room and kitchen (door at the end of the dining room)
4 = Observation deck
5 = Outside

• Ship Sinking

After you gain control (and the tutorial starts), move to the right and interact with the teddy bear.
Talk to the Paulo (sitting on the bed) about everything.
Learn that you can possess living beings that you know a bit about and thus steer them a bit in their normal lives.
Also learn that you can fully possess a person as soon as you know their innermost thoughts and feelings, allowing you more free reign and replacing their personality with yours for the time you control them.
Finally, learn that as a ghost you can only enter rooms that you have seen at least once while possessing someone.


Possess the cat that entered the room. Her special ability is high jumps allowing her to enter air ducts.
After talking to Paulo again, use the cat's special to enter the air duct. Move through it into the adjacent room and leave through the bottom.
Listen in on a conversation between Alton and Marcurio. Follow them both to the next deck afterwards.

Make sure to act like a cat to not draw any suspicion (jump around, especially try to attack vases and people looking funny at you).
Approach Alton and demand belly rubs, gaining the ability to possess him in return [actually you can possess him from the start, but meh :)].
Alton's special ability is changing the current background music (to various songs you unlock while playing).

Possess Alton and enter Xu's room. Ask why she is upset. Offer to help her.


Wander around the ship and question everyone about what they did last night; you need to talk with the following people to progress further:
Quella - wandering around all decks (including outside); mostly hangs out on the upper decks/outside though
Gwen - roaming around the lower deck; if not there she is currently in her room and will come out from time to time; also goes to the upper deck and back from time to time
Garv - roaming around the lower deck
Donovan - roaming around the lower deck; if not there he is currently in his room and will come out from time to time; also goes to the middle deck and back from time to time
Ferdinand - either on the bridge or the upper deck; sometimes also on the middle deck
Marcurio - in the kitchen behind the dining room
Darryl - outside on the deck, looking onto the sea

Return to Xu's room afterwards (Xu will let you know as soon as you talked to everybody) and watch the scene. Guess who the culprit is. Continue to watch the scene and afterwards unlock Xu for basic possession.
Xu's special ability is looking into other people's inventories while standing close to them.

This completes Chapter 1 and subsequently unlocks the achievement Ship Sinking.

• Esteem
  
• A Tale Told by an Idiot
  
• Look at Me, I'm the Captain Now
  
• The Fool

Get back to Paulo to talk with him. He has good news and bad news. Learn that you gained a special ability usable while in ghost form. This ability allows you to see into people's minds and read their thoughts while being close to them. Also learn that you can see spectral fields around you (what rooms you can enter, which people you can possess to what extent, etc.).
Finally, learn that the connection to your body gets weaker and that the course of the ship needs to be changed.

When you find the time, go to the bathroom and use your ability to detect spectral fields. Interact with what you find. :)


Go outside and take control of Xu. Enter Quella's room and talk to her about everything. Gain basic possession over Quella. Her special ability is listening to the tone of the voice of people she talks to (essentially, whenever you need someone who can make other people talk more openly and be more trusting, Quella is your best choice).

Take control of Quella and leave the room. Learn (via intercom) that you can contact the first mate if you feel unsafe. Note how Gwen reacts.


Go to Gwen and talk with her about everything, gaining basic possession over her in the process.
Gwen's special ability is to look into rooms by standing close to a door.

Take control of Gwen (if she runs off to her room, wait a while outside or alternatively use the cat to get into her room [via the air duct in the room next to hers]).
Go to each door on each deck that you didn't enter yet and use Gwen's ability. This will ensure that you can enter all rooms as a ghost.

Go to Ferdinand (usually on the upper deck or bridge) and talk to him about Gwen's worries.
Try to enter the mentioned room (Donovan's), but realize that this room is locked and occupied.


With Quella, go to Alton who is in the observation deck. Speak with him about why he is feeling so blue. Talk with him about everything.
Switch to Alton and go to Xu (in the dining room). Talk with her about everything.
Finally, as Alton, go to Quella (in her room). Possess her and talk with Alton about everything.

Gain full possession on Alton and unlock the achievement Esteem in the process.




As ghost, enter the captain's room.
Investigate the book on the night-stand and read all pages to find out more about yourself.
Investigate the book lying on the floor to gain a new save-point and progress in the associated achievement.

Go to Alton's room.
Take full possession of Alton (hold down the possession key until you are fully charged and then release while close to him) and talk to everyone on the ship about all topics you can talk about.

This is required to regain more memories and pieces about your true/former self. Make sure that no-one has a speech-bubble over their head while you fully control Alton.


If not done so already during the previous step, use the fully controlled Alton to talk to Donovan. Tell him what you know about the captain and agree to meet him outside on the upper deck.

Switch to the cat (in Garv's room) and enter the empty room right next to Gwen's. Enter the air duct and leave using the left-most exit.
In there, directly go to the exit and unlock the door from the inside.

Switch over to Gwen and go into the now unlocked room. Look at everything, especially at the key lying on the drawer in the upper right.

Report your findings back to Ferdinand and gain basic control over him in the process. His special ability is making announcements over intercom.

Continuously make announcements until Ferdinand starts to repeat himself, unlocking the achievement A Tale Told by an Idiot in the process.



With Ferdinand, go to the bridge.

Fully use every console except for the steering wheel to unlock the achievement Look at Me, I'm the Captain Now.


Finally, use the steering wheel to correct the ship's course.

This completes Chapter 2 and subsequently unlocks the achievement The Fool.

• Euclid's Elements
  
• Sanctuary
  
• Time
  
• Servitude
  
• Rough All Over
  
• Good Friends, Bad Friends
  
• The Contractor
  
• The Syndicate
  
• Implosion
  
• Wake-up Call

Return to Paulo and talk with him.
After the scene talk with Paulo again about everything (especially about you).

You will unlock the achievement Euclid's Elements in the process.



Leave the room and listen to the intercom message. Go to the middle deck - make sure to not control the cat when going up; this glitched the game for me.


After the scene, take control of Quella and go outside to talk with Darryl about everything.

While on the upper deck, take a detour to the dining room and use the tea-cup as a ghost to unlock the achievement Sanctuary (If you did not get this achievement here, you forgot the book lying on the floor in the captain's room, which you can of course still get).


Go to Xu's room and investigate what is lying on the floor. Go to the lower deck, enter Darryl's room, and completely read the book located on the table in the lower right.
Go to the dining room and talk with Xu about Darryl. Afterwards return to Darryl and talk with him about everything.
You will unlock basic possession over Darryl in the process. His special ability is taking some pills that speed up his movement, but also blurs/"numbs" his vision as a side effect.


Switch to Darryl and ask Quella about what she just said - talk about everything.
Unlock full possession over Quella and the achievement Time in the process.



As Darryl, talk to Xu about everything.
Unlock full possession over Xu and the achievement Servitude in the process.



Finally, with Darryl, go talk with Garv about everything to unlock basic possession over him. Garv's special ability is calling his cat to his side (will need some time, depending on the distance between you and Sunshine).

Switch to Garv and go to Marcurio (usually in the kitchen). Talk with him about everything to unlock basic possession over him. Marcurio's special ability is seeing/feeling/noting residues of various fluid spills. Have fun running around the ship finding them :)


Take full control over Quella and talk to everyone about everything.

When done, leave Quella's body and unlock the achievement Rough All Over. It is possible to earn this one a bit earlier, as you essentially just need to make sure that you talk with everyone while fully possessing someone (including the one you fully possessed first) - thanks to Venceremos for pointing this out :).

After you got this achievement, try to interact with people without possessing one :)

Take full possession of Alton or Xu and talk to Quella about everything (i.e. to make sure that you truly talked with everyone about everything, while fully possessing someone - this is required to regain your true self).


Take control of Marcurio and fully check the contents of both the cabinet and the fridge.
Take full control over Xu and talk with Marcurio about everything.

With Xu (fully possessed) go to Donovan and talk with him about everything. Follow Donovan and watch the scene.

Unlock basic possession over Donovan and with it the achievement Good Friends, Bad Friends.


Donovan's special ability is identifying items he can interact with.


Take control of Donovan and take the key from the drawer.
Read all e-mails on his laptop to unlock the achievement The Contractor.



Check every room and it's contents - use Donovan's special to see the items you can interact with.
Unlock the achievement The Syndicate by checking the drawer in the empty room besides Gwen's.


Unlock the achievement Implosion by checking the drawer in Alton's room.



Make sure you check every room and interact with every item Donovan's ability highlights. In the end you need to have examined at least the following:

• All camera footage on Donovan's laptop
  
• Drawer in Garv's room
  
• Cabinet in the broom closet
  
• Wall in Marcurio's room
  
• Bookshelf in Marcurio's room (specifically, Lower Shelf - Creating a Successful Restaurant)
  
• Drawers in the Captain's room
  
• Chest in the Captain's room
  
• Wall in Dining Room

As soon as you leave Donovan's body after finding all the evidence you should unlock the achievement Sense of Self. If not, you either did not talk with everybody while fully possessing someone or did not check all necessary items with Donovan listed above. Anyway, if it did not unlock, make sure to fulfill all needed requirements before continuing with your investigations.
If you do not leave Donovan's body during this chapter, the achievement will unlock automatically at the beginning of chapter 4 if all conditions are met.
After you got this achievement, try interacting with other people again while being in ghost form :)

When all of this is done, go to where the collected hints lead you as Donovan (outside) and interact with it (The wall on the left hand side; Marcurio's/Donovan's ability will help you find it as well).
Watch the scene.

This completes Chapter 3 and unlocks the achievement Wake-up Call.

• Sense of Self
  
• Trauma
  
• Empty
  
• Smiles and Tears
  
• False Utopia

In case you did not leave Donovan's body in the previous chapter you will now also unlock the achievement Sense of Self mentioned above automatically here.

After you gained control, get outside the room you start in and move to the middle deck. Watch the scene.


In ghost form, go to Gwen and listen to what she has to say. Work your way through the scene.
Tell her you are her sister.
That only the two of you are here.
That you both are far away.
That this is just a nightmare.
Gain full possession of Gwen and unlock the achievement Trauma.



In ghost form, go to Darryl and listen to what he has to say. Work your way through the scene.
Tell him you are his wife.
That this is not his fault.
That you were sick.
Gain full possession of Darryl and unlock the achievement Empty.




Fully possess Darryl and go to Garv. Talk to him about everything.
Gain full possession of Garv and unlock the achievement Smiles and Tears.




The Bracelet
As fully possessed Quella, go to the lower deck and talk with Garv about everything.
Afterwards, possess Quella normally and try to open the door to Paulo's room. Talk to Garv about everything.

Possess Garv and use his special. Possess Sunshine and get into Paulo's room via the empty room next to his. Open the door from the inside and possess Quella again.

Go into Paulo's room and check everything, especially the book and the drawer.
Next go to the kitchen and talk with Marcurio about what you found with a normally possessed Quella.
Finally, talk with Garv again (still normally possessed) about Marcurio.

The Footage
Possess Ferdinand and go to Donovan's room. Review everything stored on his laptop. (The password is lucille - you can only know this one if you read all e-mails on the laptop in chapter 3).

The Key
Fully possess Quella. Talk with Xu about Gwen.
Take control of Marcurio and go to Gwen. Switch to Gwen and talk with Marcurio about everything.
Fully possess Alton and talk with Xu about Donovan.
Fully possess Xu and talk with Marcurio about Gwen.


With the fully possessed Xu, go to Ferdinand and talk about what you just learned. Go through the following scene.

This completes Chapter 4 and unlocks the achievement False Utopia.

• Business
  
• Pleasure
  
• Living Nightmare
  
• Retribution
  
• Justice
  
• False Awakening
  
• Douce Amere

Before doing anything else, SAVE THE GAME HERE - especially make a copy of the recently saved game. If you followed the guide until here, all endings are still open to you and you are now exactly at the branching point to the five different endings.
I will thus not detail here how to get one of these endings, but would rather let you continue on your own, except for some high-level info below to prevent you from getting lost along the way ;). After you finished the game and got one of the 5 endings, check out the last section of this guide for step-by-step information on how to unlock each of the endings you didn't experience yet. :)

After the scene, do what you want. You can talk to people and explore the ship at your leisure, if you want.
As soon as you are ready to kick off the finale, talk with Ferdinand as fully possessed Xu to proceed.

Watch the following scene and make your way through the following dialogues. Generally, possess different characters to bring up various pieces of evidence you collected during the previous chapter.

Have fun piecing everything together! :)

Achievements are sorted by the order they will be unlocked as per the walkthrough above (within the respective category).

Due to the need of providing step by step instructions for some of these achievements, you may spoil the story for you if reading up on achievements before having progressed that far in the story. While I try to keep this section as spoiler free as possible, it will not be entirely feasible if I don't want to end up just giving cryptic hints ;)

You should rather use this section to check on achievements you are missing if you have already completed the game once.
If you stick to the walkthrough above you will get all of them either way - you have been warned :).


Ship Sinking
Complete the first chapter.
Unlocks at the end of Xu's and Alton's search for the thief.

The Fool
Complete the second chapter.
Unlocks after you corrected the ship's course.

Wake-up Call
Complete the third chapter.
Unlocks when your investigations finally lead you to the secret on board.

False Utopia
Complete the fourth chapter.
Unlocks after having collected sufficient evidence to formulate a concrete plan on how to solve this mystery.


Esteem
Fully unlock Alton.
After Alton's revelation about Xu, possess Quella and talk to Alton. Then possess Alton and talk to Xu. Finally, possess Quella again and talk to Alton to fully unlock him.

Time
Fully unlock Quella.
After gaining basic possession over Darryl, possess him and talk with Quella about what she said to fully unlock her.

Servitude
Fully unlock Xu.
After gaining basic possession over Darryl, possess him and talk with Xu about what you just learned from Quella to fully unlock Xu.

Rough All Over (Missable)
Piece together parts of your identity.
You will get this after you talked to everyone about all topics by having another character fully possessed (including the character you fully possessed) and have read the diary in the captain's quarters.

Good Friends, Bad Friends
Unlock every character.
Self-explanatory. You will get this one as soon as you unlocked basic possession for the last character.

Sense of Self (Missable)
Regain your identity.
You will get this after you have completed your investigations with Donovan (specifically after checking the locker in the broom closet).

Trauma (Missable)
Fully unlock Gwen.
When in your final form, move yourself (as ghost) close to Gwen and try to talk to her. In the ensuing scene tell her you are her sister, that you two are alone and that this is just a nightmare.

Empty (Missable)
Fully unlock Darryl.
When in your final form, move yourself (as ghost) close to Darryl and try to talk to him. In the ensuing scene tell him you are his wife, that it isn't his fault, and that you were sick.

Smiles and Tears (Missable)
Fully unlock every character possible.
You will get this one as soon as you unlocked full possession for the last character and regained your true self (6 characters + yourself).


Look at Me, I'm the Captain Now
Leave no switch unswitched.
When controlling Ferdinand use every console on the bridge.

A Tale Told by an Idiot
Listen to all of Ferdinand's "announcements".
Repeatedly use the special ability of Ferdinand.

Euclid's Elements
Learn more about Paulo.
After the first night, talk with Paulo about everything he has to say. Will unlock along the way.

Sanctuary
Unlock all save points.
There are three save points on board of the ship, but the last one won't be available until chapter 3. The teddy bear in Paulo's room, the book in the Captain's room, and the tea-cup in the dining room.

The Contractor
Discover one tie-in to the original Flash game.
While controlling Donovan, read through all e-mails on his laptop.

The Syndicate
Discover the other tie-in to the original Flash game.
While controlling Donovan, check all drawers in the empty room beside Gwen's.

Implosion
Learn more about one character's past.
While controlling Donovan, check all drawers in Alton's room.

Business
"There's no way this ship pays for itself."
Not strictly an ending, but rather leading to some. Will unlock during the finale by providing evidence on the human trafficking.

Pleasure
"I don't know who, but a person here is...looking for a target."
Not strictly an ending, but rather leading to some. Will unlock during the finale by providing evidence on the serial killer.

Living Nightmare

Worst Ending - you can procure no good/definite evidence at all. The key here is to bring up all evidence you have, but in an order leading to nothing conclusive.
I provide on example route below.

Step by Step:
Quella: You were in Paulo's room and found the book.
Quella: You also found the bracelet.
Quella: Marcurio gave the bracelet to the cat willingly as he wanted to keep the cat quiet
Xu: Hint on the key, that it was given anonymously.
Xu: Say that you were bribed to steal and keep it.
Marcurio: Say you were also bribed and instructed to plant the key.
Marcurio: Hint that the kid must have been taken prior to the key planting.
Grav: Say that there must be evidence around here and that the kid might have something.


Retribution

You are able to procure enough evidence to catch the serial killer only.

Step by Step:
Quella: You were in Paulo's room and found the bracelet.
Quella: Marcurio gave the bracelet to the cat willingly as he wanted to keep the cat quiet
Grav: Say that there must be evidence around here and that the kid might have something.
Quella: You were in Paulo's room and found the book.
Quella: The page torn from the book might have been important.
Quella: Is there something special about the page?


Justice

You are able to procure enough evidence to solve the human trafficking only.

Step by Step:
Xu: Hint on the key, that it was given anonymously.
Xu: Say that you were bribed to steal and keep it.
Marcurio: Say you were also bribed and instructed to plant the key.
Marcurio: Hint that the kid must have been taken prior to the key planting.
Fully control anyone: State that Marcurio was feeding people down here from kitchen stock and that the captain found out.
Fully control anyone: Ask how orders changed and what new orders were.
Fully control anyone: Point out the camera footage.
Fully control Garv: Confess everything.


False Awakening

You procure enough evidence to show that there are two things happening, but only have enough to definitely solve one of both.
IMPORTANT: This ending is impossible if you know too much. The approach to get this ending is the same as for the next and the game will force the next ending onto you if you know the crucial last piece. Ensure that Gwen does not receive the crucial piece of information after the end of chapter 4 (after setting her free), i.e. do not interact with Gwen between formulating the plan and getting to the finale to ensure you can get this achievement instead of the next.

Step by Step:
Quella: You were in Paulo's room and found the bracelet.
Quella: Marcurio gave the bracelet to the cat willingly as he wanted to keep the cat quiet
Fully control anyone: State that Marcurio was feeding people down here from kitchen stock and that the captain found out.
Fully control anyone: Ask how orders changed and what new orders were.
Fully control anyone: Point out the camera footage.
Fully control Garv: Confess everything.

Douce Amere

Fully solve the mysteries surrounding this ship and it's passengers.
IMPORTANT: The crucial information hinted at above can be obtained by taking full possession of Xu after the end of chapter 4 (i.e. after setting Gwen free) and talking to Gwen about everything. Afterwards, take normal possession of Gwen and talk with Ferdinand about everything [you should have an inner monologue of Gwen after talking to Ferdinand].
This ending is not possible without this information and you would instead automatically revert to the False Awakening ending.

Step by Step:
Quella: You were in Paulo's room and found the bracelet.
Quella: Marcurio gave the bracelet to the cat willingly as he wanted to keep the cat quiet
Fully control anyone: State that Marcurio was feeding people down here from kitchen stock and that the captain found out.
Fully control anyone: Ask how orders changed and what new orders were.
Fully control anyone: Point out the camera footage.
Fully control Garv: Confess everything.
Gwen: Point out what you told Ferdinand

I hope you had as much fun with this game as I did (and I do hope that I got the Business and Pleasure achievements correctly pinned down :D).

Let me know in case you find any mistakes :)

Thanks to Venceremos for pointing out how to get the achievements Rough All Over and Sense of Self a bit earlier :)