After an exciting (and rather drunken) fishing trip a friend posed a question that not one of 6 college educated people could answer. Do fish drink water?

Salt water fish drink.

Fresh water fish don’t.

Yeah… marine fish drink lots of water so they can absorb just the water through their guts and keep their blood salts more dilute (usually) than sea water. Opposite for fresh water fish - they drink very little, piss a lot, and absorb all the salts/ions in the water they can to keep their blood salts more concentrated than fresh water. It depends on the types of fish, but in general, that’s the story.
Get all your college-educated buddies to think back to osmoregulation from biology (if that’s what they took).