Piratecat said:
Not that much water in most of the underdark, though! 
Not true not true, gasping for air as I say it over and over again
the Seafarers Handbook in FFG's Legends and lairs brings the subject up quite proficiently.
They Called it Underdeep. They talk about it in Chapter three of the book Undersea Adventures. It includes example Adventure locations
They also have stats for Deep Drow ( or Aquatic Drow) Piratecat you gotta buy the book to understand. It is a usefull section. It has an example underdeep city named Silentdark, and a Ship example of mind flayer submercible
Adventuring through lost caves and sunken drow cities sounds fun to err scarry to me. I bought the book with high hopes, and while overall I am kinda disapointed with the quality I like its sections on Ports of Call, Ghost Ships, and The Underdeep.
It comes with a Template (Aquatic) and 4 new creatures The Abyssal Shark, the Coral Golem, The Drowned dead, and the Hippocampus. 27 new spells, New magic Items, New Domain Undersea, New equipment for undersea adventurers. 30 New feats, A new take on some old Aquatic races, and a single new PrC