A third alternative is Webasto’s Hydronic diesel water boiler / heater that will also heat your boat. Semi-trucks use these systems to heat their cabs. I have friend living is Seattle and he love it. It always him to heat water and warm the boat underway without the use of his engine.
Also I have been told but never verified that some insurance companies will not insure boats with propane heaters or “on demand” water heaters. Might want to ask, just in case.