I've seen several water slides where you go down it on your back, with your arms and legs crossed (or rather supposed to be crossed). I went down one like that in "water world" which used to be in Houston.

I don't remember the size, but at the end of the slide there is about 2-3 inches of standing water in the chute which I guess serves as the "water brake". I was wearing some long swimming shorts, but after hitting that water brake butt and feet first, once I stood up it looked like I was wearing a speedo since the blast of water going between my legs gave me the worlds largest wedgie.

Never rode a slide like that again. I used to stand at the end and laugh at the people getting off and having to pull their cloths out of their crotch as they walked off like penguins.