Check this thing out
link to article 
A high-speed land and water vehicle drives on the River Thames Wednesday, Sept. 3, 2003. The Aquada is designed to reach speeds of 100mph on land and over 30mph on water and can switch between the two surfaces at the switch of a button. Gibbs Technologies, who designed it, says that no other road-legal amphibian has managed to exceed 6mph on water.
