The water jetpack thing was on discovery channel a while ago. It was a standard jetski powerplant inside the little towable, with a big ass hose connected to the nozzle. Really pretty simple, I can't imagine it would be that difficult to make. Even the most modest jetskis out there today can produce hundreds of pounds of thrust....easily enough to lift a person straight up.
What would really sell like crazy is a bolt-on adapter kit for a stock jetski.