This may be one of the coolest vehicles I've ever seen. The BiSki, which looks like a fat motorcycle, can transition to water and turn into a jet-ski, nearly seamlessly:

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Designed by Michigan-based Gibbs Sports Amphibians, the BiSki can do 80 m.p.h. on land and 37 m.p.h. in the drink. Should you need to go slightly faster, the BiSki has a sibling, the TriSki, which is good for 85/45 m.p.h. on land/water:

And if you need more stability on land, there's the QuadSki, seen here in police livery for land/water patrol applications:

While both the BiSki and TriSki are prototypes, GSA's website lists service agents for the QuadSki, which is apparently in production.