The Di-Cycle, by GBO Design, bills itself as an amphibious bike for the city of Helmond in the Netherlands, as the city has as many watery channels as it does roads. Riders would be able to skim over the water as easily as zip across the land. If this thing really worked, that'd be awesome, but I've got a few gripes with dicycles, like how some of them flip you upside-down a few times when you're riding. If it could keep me upright and biking across the water, I'd be all for it.
The Di-Cycle is just a concept, though dicycles themselves are pretty commonplace as hobby projects.