LogFAQs > #880239839

Tails, Claire, Juliet, Geese: All worthless

Tidus: Obviously really good here

Neclord: Vampires don't typically do well underwater, do they? Also, pretty sure a swarm of bats can't swim too well, so he's gonna be a sitting duck

Crono: Probably can't maneuver too well either. Also, Crono isn't the type to want to even possibly hurt his friends, so he might be hesitant to use his lightning magic as there's no telling how it will function underwater.

Nisha: Potentially not worthless? Borderlands guns are crazy enough that they might function underwater.

Frog: Probably just as good as Tidus. He's a fucking frog, and water is his magic element. Plus he has a lot of healing. Shouldn't be hard to get a blow on Neclord with the Star Dragon Sword

Kaos: Amazing. One of his attacks is literally summoning Touhou-style waves of energy sharks (https://www.youtube.com/watch?v=9QZK_YSNX2s&t=14m51s). He's enough of a cartoon villain that he'll want to throw them out nonstop because everyone is underwater. And since no one on the enemy team except Tidus is good at moving here, they'll be torn to shreds. He can also take a ton of punishment. His boss fight takes forever, even if you don't think he can heal himself here.