Just to throw in a monkey wrench - FW fish do not need to drink and do pee a lot, SW fish must drink lots and don't pee much. ("Drinking like a fish" obviously refers to SW critters).
We'll deal with that one another time, but it is the other side of the same coin.
What's to deal with? It's the same thing (SW vs FW) except the movement of everything is reversed. High concentration ---> Low concentration, this is the law. If the fish needs something or needs to get rid of something against this gradient, it must physically do so.
Oh forget it...when you don't act serious, no one takes you seriously
