Verified answer

Hmm. I've heard of an OVA called Blue Submarine #6 that might fit the bill.

The Little Mermaid

ehh, not really. maybe the fishman island arc in one piece?

http://onepiece.wikia.com/wiki/Fishman_Island_Arc

and also the movie ponyo i guess. they both take place underwater(one piece does too, at least in this particular arc) but neither are at all similar to firestorm.