You have to fight a battle underwater. You can only do this in the world of balance, or you lose your chance forever. Go to the Veldt cave where you find Gau's treasure (diving helmet) and then continue through the cave to the dive point (it'll lead to Nikeah). Dive, fight, get Water Rondo. Oh yeah, and if it wasn't obvious, you need Mog in your party.

FQ, I got nothin.