SSE Removing water from an area

[Katarsi]

I'd like to remove water from the Ragged Flagon - from the entire Thieves Guild underground space, actually.

It goes against sanity that people live in an underground cavern, with no sunlight but surrounded by water. They even sleep down there! I can't even count the amount of illnesses that kind of environment would cause.

 

So how do I do this?

Removing water is the first part. Putting something else in its place is the second, but that's later.