"Tank" style hot water heaters will often use engine coolant to heat the water. In the hot weather the water will stay warm in the tank for hours. So, it's "free" if you time it right.
If one does not shower often on board or sail in cold climates, then a water heater of any type is a waste of space.
I will admit they are nice to have. My last boat had one till it died, then it didn't. I missed washing dishes in warm water, but, I liked the extra space for batteries better