The Waveboat attaches to a Jet Ski in under 60 seconds and effectively turns your personal watercraft into a boat. It costs $8,000 but is currently only available for attachment to Yamaha Waverunners. That may change if they catch on. And I definitely see that happening. Just look at all the extra beer storage space it provides. You could fit kegs up there.
UPDATE: The Geekologie writer does not condone the act of drinking and boating. Despite the fact that they were seemingly made for each other. But trust me, I wrecked my buddy's boat. In my defense though that mermaid really did come out of nowhere.