Here's something you could have a lot of fun with:
http://www.aquada.co.uk/mediacentre/quadski_low.phpOr if you prefer a bigger version:
http://www.aquada.co.uk/humdinga_4x4.phpThe developer doesn't manufacture the gear but seeks to license it. They've started going after military applications; I'm a little surprised nobody's picked up on it for civilian use as the technology's been around for a few years now. The cost is significant but not astronomical. I think they were looking at 175 grand or so for their 2-seater Aquada; plenty of people spend that on a Bentley or Ferrari, and I know which I'd have.