I think Super Mario Bros might have been the first. Might depend on what counts as a “platformer” or a “water level”.
Jump Bug (late 1981) is the earliest. It has an entirely underwater level, and the car moves more slowly in it. It was also one of the earliest platformers, beaten only by Donkey Kong (mid 1981) and a couple others.
Super Mario Bros. (1985) introduced a significantly different mechanic for its water levels from the rest of the game, specifically swimming, and it was about a bajillion times more popular than Jump Bug. But it wasn’t the first.
I’m worried that you may be a time-traveler with a gruge.
Sub Hunt, Frogger, White Water
Water, sure, but none of those are platformers.
I’d call frogger a platformer, but the water aspect is only aesthetic. So disagree with OP on that but for different reasons.
Anybody have one that predates Pitfall II: Lost Caverns (1984)?
Jungle Hunt was '82.
Huh… I see swimming through water in this video, but I don’t see platforms.
A lot of water levels in Platformers have swimming instead of platforms like the entire rest of the game does