It sucks to be stuck in the ocean with no places to use flying mount. After Blizzard's new water graphic, there's a solution(oh God, I sound like a TV-shop guy).
This little trick might save some time for some of you guys.
When in the water, swim to the surface(where you start regaining breath). Begin summoning your flying mount. In the end of the cast, jump. You should now be able to flee from the water :P (not tested with druids).
Alca