That would imply no hot water as soon as you leave shore power or you would need to fire up the genny whenever you wanted some. Personally I have never run out of hot water at the dock even though we both take showers. Those small water heaters really heat up fast. Mine is only 11 gallon.
It boils down to simple physics. In order to heat cold water as it passes through, you need a lot of juice, much more so than a standard water heater. You will not find a low draw unit that is worth installing.
That being said, I have experimented with the propane variety off the swim platform. Now these things really do work well. Played with it in the garage a few years ago. 54F cold water in, 100+F out.

Adapted a 1lb bottle to it to keep it compact which is good for about 3 long showers.
Really wish I could install something like this and draw straight from the lake as the water here is crystal.
This is the unit I have. The girl does not come with it.
https://www.ebay.com/itm/8L-Gas-Tankles ... Sw3ydV3UEr