Aeolius- I liked that thread about Underwater Ambience.
I have to agree with Lycanthromancer about using Tangerine Dream as background music. The first to spring to mind, of course, would be Underwater Sunlight.
[ame=http://www.amazon.com/Underwater-Sunlight-Tangerine-Dream/dp/B00004NKBE]Amazon.com: Underwater Sunlight: Tangerine Dream: Music[/ame]
In addition, you might try the soundtrack to Cirque du Soleil's underwater show, O.
[ame=http://www.amazon.com/O-Elise-Guay/dp/B000A2H9WU/ref=sr_1_1?ie=UTF8&s=music&qid=1244613777&sr=1-1]Amazon.com: O: Elise Guay, Rhéal Jutras, Julie McInnes, Benoit Jutras, François Jutras, Roxane Potvin, Vovo, Stéphane Gariépy, Toumany Kouyaté: Music[/ame]