Jim,
If you had an L-88 Amphicar and lived in Detroit, you could actually take take the car to Niagara Drag Strip in New York, by way of Lake Erie.
As long as you kept the revs up, the nose would stay up. Drive the car to the docks in Western New York, and on to the Drag Strip.
Not sure on MPG in the water. One thing to be aware of, if you stopped in the water, the car would drop like the 'Titanic', nose first
.
So, when you entered the water, you had to be going 100MPH to keep the car afloat
At least we can dream a little..
Paul