Water Rondo is one of Mog's Dances, and all you have to do is win a battle with him in your party in either the Serpent's Trench or the Lete River - WITHOUT USING DANCE. He has to fight the battle normally. There are NO WATER BATTLES in the World of Ruin, so get it now.

dances are learned in -
cave (starts with)
water
mountain
plains
desert
forest
town
snow (can't learn until wor)

By forbidden island do you mean the Floating Continent? Yeah, Behemoth is rough. You might have to level up some more... if you can find a save point on the island (I don't remember if there's one at the beginning) you could use it as a sort of base camp... otherwise, I'd level in the Thamasa cave. Don't bother with Triangle Island, Intangir's not worth the trouble.

Also, if you have a hard enemy on the Floating Continent that doesn't use magic (the big guy in the monster-in-a-box comes to mind) try casting Phantom, but it will hurt like hell if you're wrong and he does have a magic attack up his sleeves.


EDIT: SHUT YOUR HOLE ESCOBAR THERE ARE NOT >O

EDIT2: And try to have equipment for Shadow and Celes sitting around., ready to equip~ And do NOT bring Celes in your group of 3 - it'll make it easier on you later.