Can you even buy an electric marine hot water heater that isn't pre-plumbed
with a heating coil?

In cases where such a heater is installed in a boat that doesn't have a closed
cooling system, I usually see the fittings for the coil capped off.

I have seen some household type water heaters stuffed into the bilges of
Cruise-a-homes and outboard powered boats with shorepower conections. The vast
majority of time they rot out very prematurely.