Since it keeps coming up, lets clear this bit up:
Its fine to not apply Pokemon battle mechanics to Bowser, he's not a Pokemon. So we can't assume water will have any extra effect on him. Blastoise however, IS a Pokemon, and a water type, so fire doesn't do shit to him. That's just one of his natural properties, so no matter what rules we go by, it holds true that fire won't hurt Blastoise (or only hurts a tiny amount, if you wanna get technical here).