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
(Downsized Mechanical Engineer)