We use an "on demand" Coleman water heater. Uses propane. Works great! Really only use the coleman during winter camping as the simple solar showers work really good in warmer temps. Heads up about solar showers they can become scalding hot. :cry Once the reach about 100 degrees its better to wrap in towel and leave out to stay hot, not too hot.
link below should show you about the coleman. Thing I don't like about the coleman unit is a "single purpose" relatively large item on a small boat.
