Wizards wave their hands and say something like, "Whatever, space is an ocean."

The Expanse makes the observation that accelerating your ship at a substantial fraction (or multiple) of Earth gravity gives you that same degree of artificial gravity, only, you have to orient your decks the right way. Down is towards the engine.

Of course, this in turn makes Star Trek (and Star Wars, and Firefly...) look even sillier, because flying in a direction perpendicular to your deck layout means you need two magic gravity fields — one to cancel out the engines and one more to give your crew a place to stand.