Kanoba:
ex_underwater.wav

OpenGameArt:
watersplash.wav
watersplash2.wav

Scorched 3D:
splash.wav

large_water_explode.wav - Open LieroX & Scorched 3D

Sphiloth aka. Alcur of the Zero-K community (http://zero-k.info/):
underwater_resounding_thump.wav
underwater_resounding_thump_short.wav