I think an awesome game would be wave race....
Think about it for a moment. It wouldn't use the style of input such as described here "hold the wand sideways, gripping it like you would a water ski rope. Rotate it back and forth like a steering wheel." That's perfect for a car-racing game, but I think theres a better way to control a jet-ski.
Imagine holding the controller face-up like a remote is normally held. Also imgaine a mini-jet-ski racing standing on-top of the controller near the center and you'll better comprehend the scheme. Anyway, say you want to turn left, you rotate and tilt the controller left. This would simulate the jet-ski skidding on water. I'll further describe the process: You rotate the controller so the top of the controller is now at some degree to your left. You, at the same time, tilt the controller, so it is at some degree on its side. It would mimmick exactly what a jetski does.
You can also dive into the water by tilting the controller forward. Flips and barrels could work (although I don't have a really intuitive idea at this time).
WaveRace is already my favorite racer, and with this controller it would be insanely fun.