It seems to me that hot water is the last great frontier. Here on the new boat we have more than adeqate solar, also wind and a high output Balmar alternator. Not to mention the towable jobby if things get really desperate.
The reality is that we can survive without running the engine at all, except when we need to move the boat under power or if we have a prolonged cold windless cloudy rainless spell.
Nonetheless if we want to have hot water we need to run the bloody engine an hour a day. It somewhat beggars belief that no one has managed to come up with a half way decent 12v hot water heater. I guess there is simply not enough demand to justify the research and development into more efficient heating elements. Thats about the only thing I can think of.