In the Imperial City Waterfront, there's a docked boat that's been outfitted as an Inn, called the Bloated Float. If you stay there, it has it's own quest, but I've noticed there's a room called the "Bloated Float Private Room". The door seems to always say it is sealed? What is this room for? Is it related to a quest? As far as I know, it's not related to the bloated float pirate quest that triggers when you sleep there? UESP says that the door is sealed until you are doing the related quest, but I'm not sure I've been able to open it even when doing the quest? Is there something or someone in there which is related to some quest and which would spoil the quest if you got in too soon?

Yes it a part of the quest and it will never be open

I've done the bloated float quest several times on several different characters, and don't ever remember that room playing any role in the quest? Perhaps it's something the devs never implemented?

if you take the leader to jail she gets locked in that room

Oh, wait, there's a way to not kill her? I always just ended up killing her lol. Never figured out how to talk her into going to jail.

If you show her that you know a lot about their meetings, their aims etc. through the dialogue options she will surrender to you, give you her blade and the Inn owner will ask you to escort her to the locked room.

Interesting, I've always just killed her myself. I really like that sword of hers, Absorb Fatigue is a good enchantment to have on a weapon.

She'll give you the sword if she surrenders.