Our engine-heat-exchanged water heater died a few years ago and I switched to a tankless propane on-demand system. Along with solar panels, it enables us to not have to run the engine except for propulsion. The unit we bought isn't cheap (~$1500) but it is an elegant solution to providing simple showers. I set the thermostat to ~100F with a single output line so there is no water wasted trying to mix hot-cold for a comfortable temperature. One short lead goes to a cockpit shower, the other short lead goes to a head shower. We've been very pleased with it, and hardly notice the propane usage.

http://www.precisiontemp.com/showermate-m-550-marine-tankless-water-heater/