Yeah, I've been in Illinalta's Deep before picking up some supposedly related quest to it. The place is covered with water, and ghosts from ex-prisoners are all around. poor guys were left to drown by their guards ...

Nope, that's Abandoned Prison.

The abandoned prison totally bugged for my first toon. Can't go there at all. The others... trying to recall if there's a reason to go there, lol.

I never found a quest sending you there - and the mod I use (http://skyrim.nexusmods.com/downloads/file.php?id=9720) doesn't pop a quest name when discovering it....