Now you're contradicting yourself, first you say its not usefull outside of water, now you say the temperature he generates is almost a sun lvl temperature and say the MC can't create such mechanism or adaptation to release a sun ray/bean...
I could have used a Bombardier beetle but that is not really fire but more of a acid level mechanism...
All that aside you didn't exactly said that but implied it, i understand the need for "realism" but the same can't be said on a world where a guy can transform into basically anything or anyone...
You need to decide first what you wanted to do with your comment, was it a geek comment trying put logic into a world where a kid can create metaphysical rules or what?
My comment never had any realism in it, i just picked a creature that he didn't use yet that can generate certain effects to give the MC a new power, nothing else