We have friends on two different sail boats that have propane hot water heaters. They like them a lot. They are installed in the engine room. They have CC 41í sail boats here in the states. We asked our insurance company if it would be OK and they sail only if we mounted it on the rail (or such a place). So that is the rub. Nice hot water and no insurance or well maybe when we go cruising we will not be able to afford insurance so .. well propane hot water heater
They are popular in Europe I hear.