Amphibious "Sealegs" Boat

This boat will do over 60 mph in the water. The bad news is it only does about 6 mph on land. This amphibious vehicle is optimised for boating.

The Aquada from Gibbs pictured below is pretty fast on land or in the water, but is obviously not as functional for boating as the Sealegs in the video above.
A designer must make compromises when designing a multi-purpose vehicle. Vehicles designed to function as airplanes, boats, and automobiles are particularly difficult, due to the multiple constraints introduced by each of the three functions. As I have mentioned multiple times, my ideal vehicle would be legal and handle well on the highway (and off-road), travel swiftly and smoothly on or below the water's surface (down to 300 meters), and fly above 20,000 ft altitude with a range of 1500 miles. I would also like enough room to sleep comfortably in the vehicle. (but not while personally piloting the "plane."

With today's materials and propulsion technologies, such a vehicle is impossible. With a timely assist from nano-materials and molecular assemblers, along with advanced propulsion technologies, the vehicle could be both light enough and strong enough to fill all those roles.