Fire is important but, borrowing from Jared Diamond, I think agriculture/domestication is the defining technology that enables population density and growth. A steady food supply enables density, which leads to exchange of and competition in ideas. e.g. compare a city with 100,000 citizens vs. a hunter-gatherer society.

Fire is related to agriculture but maybe not essential. It could be some other kind of chemical reaction that occurs outside the body.

It's hard to imagine how descendants of orcas could create agriculture in the ocean, but not impossible. I mean humans have domesticated fish, shrimp, mollusks, etc. for farming. And probably some marine plants.

It does seem like shallow water is probably the best bet, but maybe that's my lack of imagination.