There is no body that determines the criteria and speed stats for amphibious vehicles. However, that doesn’t mean people aren’t building them. Currently, it appears there are two competitors for the title.
The Python in one word is the ULTIMATE amphibious vehicle. It is a hot rod on the water, excess of 60 mph water speeds, and a hot rod on the street, mid 12 second 1/4 mile times and 0-60 mph in 4.5 seconds. The engine is based on the famous aluminum LS Corvette power train. Enormous Horsepower!
Our next contender is The Sea Lion designed and built by M. Witt.
The project is aiming for the land speed record for amphibious vehicles, with a current top speed of 125mph and a transaxle geared for 180mph. The water speed is approximately 45mph, but there are hopes to increase this.
Either of these vehicles can be yours for the low price of ~$200,000. It would be an ideal solution for an Island dweller like myself. As soon as someone gives me a ticket and I win over $10m, I fully intend to pick one for myself.