I had two propane demand hot water systems on a 41' Island Trader ketch. One for potable water, and one for raw water. Premise was that I could take a long hot shower with raw water, then rinse off with hot potable. Worked OK, but had to keep the sensors clean, clean, clean.
The demand hot water systems require power, water flow, and propane to ignite. The propane and power were there whenever the system was energized. In theory, only when you opened a hot water valve, did you have flow. - And there's the danger. If the sensing unit is dirty, corroded, got a speck of junk in it, then it senses flow, and the heater lights. - No flow... superheated water, steam, and BOOM! - I was lucky that the hoses just got hot, softened, and slipped from under the hose clamps. No injuries or damage, just some water spraying around the cabin.
I am replacing the water heater on Last Tango with a 120 VAC / heat exchanger system. Much less worry and risk. Save the propane for the Barbee.