Hum, I think you will find that the
electric instantaneous type water heaters would draw way too much
power. I've specified some for
commercial buildings in the past but the
electrical engineers hate them. For example a 4KW unit might give only a 30-40 degree F. water temperature rise, with 50 degree incoming water that would be about 90 degrees, or a fairly cool
shower. Really would require a 6-8 KW to do quickly, what the standard heaters do in hours.
The standard 30amp shore cable is only 3.6 KW (In the US anyway).
You might be able to rig up an extra 12V
water pump that would circulate the cold water in the hot side back to the water tank. There is something like this for light residential that recircs the hot water side back via the cold side, but nothing in a 12V
motor.
Beni
(Downsized Mechanical Engineer)