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If the water is solely there the protect you from getting hurt or killed in a tough platforming sequence that would fall under Anti-Frustration Features
This signature says something else when you aren't looking at it.What the OP is talking about is probably common enough to deserve its own trope if it doesn't already have one. It's not necessarily water, but rather any such walk of shame where you fail the platforming challenge. Rather than just kill you, you're dropped into a little closed-off area that funnels you back to the beginning of the platform puzzle and forces you to try again. If anything, this even more common in non-platforming games. FPS and 3PS games in particular do this a lot— Tomb Raider and Portal come to mind. It's less frustrating than starting the whole level over again, I suppose. Then again, they could just set a checkpoint at the beginning of the platforming area and kill you, like an actual platforming game would do.
I suppose it's to give you time to cool down and think, which is fine for games where those elements are done relatively well. When they're not, however...
This usually shows up earlier on, so you can get used to the idea of platforming, before they take away the training wheels and start filling the space below with acid, lava, Grimy Water, bottomless drops into the distance fog, etc.
TLP it?
Edited by Unsung^ When used as an easier version of a later hard challenge, it's an Antepiece.
Edited by HeraldAlberich

A type of Soft Water that exists in platform games, in which there is little to no reason to ever be in the water unless you fall off a series a platforms. There are no secrets in the water, and all you can do is go back to the start of the platforming section and try again.
An example of the distinction: In Mario 64 all the water is Soft Water, however, the water in the carousel area of Boo's haunted mansion serves no purpose but to prevent you from taking fall damage. The only thing there is to do in this water is to go back through the basement area to where you were before you fell.