The Amphicar is rear engined. It wouldn't get nose heavy. The ZL-1 option would save some weight here
. At 430 factory rated HP, 10 times the 43 HP Triumph they came with factory.
A friend of mine restored one of these for a customer about 20 years ago. I don't know if he water tested it?
There used to be a few of them in my home town with a recreational lake here. Some of us were camped out along with a couple of kegs just below the Gavins Point Dam on the Nebraska side of the Missouri River. An Amphicar came down the park entrance road with a county sheriffs car in pursuit. The driver with his passenger dived into the river at the boat ramp at about 40 mph. It kind of nosed in but bobbed up, they engaged the props and went downstream.