Cerulean Seas will provide excellent rules for underwater everything. They have several other supplements with underwater psionics, monters etc. I hear the quality is excellent. Check out the reviews
Also check out Sunken Empires by Open Design.
I wanted to recommend raging Swan Press' awesome GM Miscellany material, but they do not have any underwater material for that line ...yet. Their Adventure Dark Waters Rising is set underwater though.
|Michael Sayre Organized Play Developer|
Cerulean Seas is great for underwater stuff, yeah. XD It's kind of their thing.
The Cerulean Seas Campaign Setting is actually the book that converted me over to Pathfinder, and was the first book for Pathfinder I ever bought (before any other Paizo books, including the CRB!) I like it a lot and have used it, and the other books in the series, many times over the years.
If you have any qualms or concerns about whether or not 3pp materials will be a good investment for your group (fairly common even if it saddens me a little), or even if you just want more aquatic goodies, you should also check out Aquatic Adventures from Paizo.