This is one of those things that seems like it should be obvious, but isn't stated anywhere in the rules, so far as I can find. Can amphibious creatures (read; things that can breath underwater) speak underwater?

If so, how? Can they only speak Aquan, a language literally created on a plane where there is only water, or are any languages viable? Would it work like dolphin speech, or is it like Drow Hand Sign, and doesn't actually use verbal components?

If there actually Are rules about this, stated in the Pathfinder books, please say which book and what page number. If it's something mentioned in fluff or a novel, please give the name of the book.