There's a really cool company out in Alameda, CA called Kai Concepts. One thing they make is the Kite Boat. I was fortunate enough to be contracted on as a part-time developer a couple years ago. What I made was a new pump board that inflates and deflates the internal bladders of a large kite pulling a boat. The regulating of the internal air chambers allows the angle of attack to be changed, thus raising or lowering the kite. It was a great learning experience.