Oblivion:An Unexpected Voyage

Quick Walkthrough
# Sleep on The Bloated Float; you will awaken to find it hijacked.
# Speak with Lynch about the gang now commandeering the inn and free the bouncer, Graman gro-Marad.
# Speak with and kill Minx.
# Speak with and kill Wrath to obtain his key.
# Make your way to the cabin to find the leader of the gang.
# Decide whether to kill her or negotiate with her. Negotiate successfully and she'll surrender herself and her sword; Ormil will apprehend her.
# Rest again in your room: the ship will be back at the dock when you wake up.
# Speak with Ormil about the gang to receive your reward in gold.

Detailed Walkthrough
Go to The Bloated Float Inn in the Waterfront district of the Imperial City. Speak to Ormil, the publican, and rent a room for 10 gold. Head below the deck to your new room. Now you must sleep, but before doing so, move the pitcher to find a bag with some gems (see Notes section below). When you wake up, your journal will update to tell you that the Bloated Float has been put out to sea. 
Lynch
Leave your room and you will immediately run into a bandit named Lynch. Talk to him (the correct options are the bottom of the two responses each time) to obtain some important quest-related information:

 
 
 
 
 
 
 
 
 
:* -colored dialogue indicates that the conversation ends with Lynch attacking you.
:* -colored dialogue indicates that the conversation continues.
:* -colored dialogue indicates that the conversation either begins or ends there and that no other option exists but that one.

You've now learned that the Blackwater Brigands, the gang that has taken over the Bloated Float, are a small group of four that formed three months ago. Moreover, Lynch just told you that their leader is holding the ship's captain hostage, and he himself is guarding the sole other person, the inn's bouncer, in the storage room nearby. 

Kill Lynch to get the key to the storage room. You'll also find his instructions. Use the key to open the storage room and free the bouncer, Graman gro-Marad. He tells you he can steer the ship back to the city, but that you'll need to clear out the remaining bandits and unlock the top deck for him to do so.

Minx
Up the stairs, you'll meet the next bandit, Minx. Talk to her (selecting the bottom dialogue choices again) to learn some more important information regarding the Bloated Float's unexpected departure:

 
 
 
 
 
 
 
 
 
:* -colored dialogue indicates that the conversation ends with Minx attacking you.
:* -colored dialogue indicates that the conversation continues.
:* -colored dialogue indicates that the conversation either begins or ends there and that no other option exists but that one.

Minx revealed some important information quite by accident: the most useful of it being that Selene, the gang's leader, is holding Ormil hostage in his own cabin.

Wrath
Killing Minx will net you the key to the upper deck, so go up the stairs and unlock the door. You will come face to face with the third bandit, Wrath, on the top deck. Talk to him, being sure to choose the bottom dialogue options:

 
 
 
 
 
 
 
 
 
:* -colored dialogue indicates that the conversation ends with Wrath attacking you.
:* -colored dialogue indicates that the conversation continues.
:* -colored dialogue indicates that the conversation either begins or ends there and that no other option exists but that one.

Wrath is perhaps the most conversationally lucrative of Selene's three henchmen, as he tells you that the Brigands took over the ship in order to find a highly valuable "Golden Galleon", which they planned to fence upon returning to their safe house in Bravil.

Kill Wrath to obtain the key to the captain's cabin where Ormil, the innkeeper, is being held. Head back indoors and tell Graman gro-Marad that the coast is clear. He'll head to the top deck and stand by the wheel, but refuse to return to port until Ormil has been rescued.

Selene
Inside the captain's cabin you'll find Selene, the leader and final member of the Blackwater Brigands, holding Ormil hostage. You now have two choices: kill her, or talk her out of fighting by using the information you've learned from the previous bandits. If you decide to kill her, skip to the Reward section below. If you decide to spare her life, you'll need to once more choose the bottom responses when you speak to her. The specific answers to select are bolded and in green below:

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
:* -colored dialogue indicates that the conversation ends with Selene attacking you.
:* -colored dialogue indicates that the conversation continues.
:* -colored dialogue indicates that the conversation either begins or ends there and that no other option exists but that one.

Once you tell her that they're dead, she'll surrender and hand over her sword, the Blackwater Blade. Talk to Ormil and select the "Selene" option; he'll instruct you to escort Selene to a room on the inn deck where she'll be locked up.

Reward
Return to Ormil for the back-story of why the bandits hijacked the ship. Business had been waning, so he invented the story of the "Golden Galleon," a statuette of a ship made of solid gold. He spread rumors that it was hidden somewhere within the Bloated Float, in order to attract the business of adventurers and the curious. The Blackwater Brigands had hijacked the ship to hunt down this fictional treasure. After Ormil finishes explaining, head back down to your room and rest again. When you wake up, the ship will be back at the Imperial City docks. Talk to Ormil for your level dependent gold reward.

Gold Reward


Notes
After renting the bed there is a 0.1 hour game time delay before the quest will start. Thus, you need to wait at least 12 seconds (real time) before sleeping in order to trigger it.
Make sure to loot everything you want from the Brigands before heading back to your room to rest for the second time, as all of the corpses will vanish once you have returned to the Imperial City. Alternatively, you can remove it from the corpses and drop it onto the floor. After the corpses are removed, this loot will remain. Note that if you drop the loot in Ormil's cabin, retrieving it will be considered trespassing after the ship has returned to port.
In your cabin on the table near your bed, there is a secret sack inside the clay pitcher. Move the pitcher to one side to find a "hidden bag" with a few jewels inside. You can retrieve the loot even if you have not started this quest.
If you are trying to control when you level up, note that you have to use a bed twice to start and finish this quest. This may also mean missing five potential skill training sessions if you have more than one level-up due.
Obtaining Ormil's Cabin Key earlier from Ormil will not work. This is because there are actually two separate keys sharing the same name. One is the default key, the other is only used for this quest.
There is no way to jump off the boat while it is at sea, nor to fast travel.
It is possible to complete this quest without ever having to engage in a fight as long as you have an invisibility spell. Cast invisibility before Lynch starts a conversation with you. After a couple minutes of running around looking for you he will unlock the storage room door on his own. Graman will then punch Lynch to death while he stands around looking for you. Go upstairs and talk to Minx, and then run back down and cast invisibility. Graman will pick up Lynch's sword (or any other weapon that you drop on the ground), and kill Minx while she too stands around looking for you. Use this same tactic with Wrath. For Selene you can either let Ormil get his revenge and punch her to death, or you can lure her up to the wheel and let Graman take care of her quickly.
Even if you don't free Graman, he will find his way up to the top deck and act as if you freed him.
Strangely, killing the brigands before they talk to you (using one hit kills, frenzy, provoking them, etc.) will enable the journal to update with all the information you would get from talking to them, even though you never heard them tell you anything.
Be aware that Ormil and Graman gro-Marad are Waterfront residents, and so accidentally hitting them during combat will trigger expulsion from the Thieves Guild. In the cramped quarters of the ship's rooms, it is easy to accidentally hit them with a stray fireball, so beware.
The Dark Brotherhood quest A Knife in the Dark will not trigger when sleeping in The Bloated Float Inn before An Unexpected Voyage is completed.
Ormil may require you to pay for lodgings a second time in order to complete this quest. Be sure to have an extra 10 gold available, or items to sell in order to obtain the needed gold.
During this quest, your character is given the opportunity to claim, unsuccessfully, to be the ship's cook, mimicking dialogue from the Steven Seagal movie Under Siege (see here).
On the inn deck there is a door to a room to which you can lead Selene to; this door cannot be entered normally. However, on the PC you can use the console command 'activate' while referring the door and thus enter the room (which appears in a different map). However, you will not find anything useful there, nor anything that can be sold. Selene will not actually be present in the room.
If you choose to kill Selene, be prepared for a difficult fight if you are a low level character whose primary method of combat is melee. Her Blackwater Blade will quickly drain your fatigue and keep it at a constant zero, which effectively renders your character immobile and unable to fight back. Restore Fatigue potions help alleviate the fatigue drain from her weapon.
Some unused voice files hint that the Blackwater Brigands were meant to be called Blackwater Raiders at some point in development.
The Blackwater Brigands also appear in the Dead Man's Dread Creation for Skyrim Special Edition. A note in that game mentions an old Imperial woman speaking of the Golden Galleon, implying that Selene survives and refounds the group.

Bugs
 
 You can fix this by making the Elven Maiden your current quest, using the console command   and finishing off the Elven Maiden quest, making An Unexpected Voyage your current quest again and completing it and then using the console command afterwards and resting in the bed to get everything set straight again.
 
 
 

Journal Entries