I just have this image in my head of a sunken city or something where everything is “sunken”. That is, the world is just water, but the tops of buildings and trees and whatnot poke out at the top.
Those are your platforms/islands/only usable areas.
And maybe staying too long on a single one will make it fully sink, or all the pieces drift away following some system/mechanic/rules.
Now do some action game, or a sport, or a platformer, in a world that’s completely like this. Where you constantly have to be precise with jumps to not fall in the water, and never stay anywhere too long because that thing will definitely sink into the water then.