Vintage gaming community.
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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.
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
Sure, but I don't see platforms anywhere in that particular game.
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.
Upvoted, but I'm not sure frogger counts. I haven't heard of the other two.
I'm worried that you may be a time-traveler with a gruge.
If you mean "water level" as in where for an isolated level you're swimming, but not the entire game, super Mario Bros might be it
Released in 1985, about a year and a half after Pitfall II.