I remember trying windsurfing a couple times on a friend's rig, and I got to the point where I could stand on it and get it to stay upwind, then I fell off the back of it one time, came up out of the water, and the mast came down in a diagonal chopping motion, hit me right in the side of the jaw and damned near knocked me out cold. From that point on, I was done with windsurfing. These guys doing it in giant waves, where you could end up tumbling in your gear in the equivalent of a giant horizontal washing machine, with virtually every part of your gear hard enough to potentially KO your ass underwater. I'll pass!
I think the jet thing would be a blast to do for about an hour, assuming someone else shelled out the $$ to buy it and fill it with gas, towed it to the location and set it up for me. After an hour, it would either be out of gas, or I would have died of sheer boredom. Let's face it, the three tricks you can do with it are dolphin diving, corkscrewing around, and flipping. How many variations of that can you do before you're done with it, and all of a sudden the money you spent buying it and filling it with gas and the time you spend setting it up all seem a little silly?