Well, poncho is the first thing that came to my mind, though it’s a real stretch. You really have to see the picture to get the idea. But lets for a moment ignore that it looks nothing like a poncho and continue with the explanation, assuming that it does.
You fit the poncho around your torso and fill it up with air, though presumably you could wear a pre-inflated version, but that’s beside the point. You then jump in the water and get yourself towed at breakneck speeds, for thrills galore.
The device is ribbed, so you get some amount of control over where you get to point yourself to. The only drawback I can see with this design, is the notion of just what might happen were you to decelerate suddenly, what with your head being the first likely thing to impact.