We have a Bosch on Demand propane
heater aboard. It's been with the boat since 1991. It works great. This is a ventless unit that has an Oxygen Depletion Sensor so it needs no outside vent or chimney. Most other units do need one so it is a nice feature if you can install it without a vent. That said I always open the port when using it as the heat is significant. This aids in drying out the shower
since the unit is literally in the shower
The unit has an inline shutoff for the propane so you can take it apart if need be. It seems to make enough hot water that you could drain the tank and still have warm water. That's probably the bad news as you really could take a very long shower until you ran out of water or propane. You can adjust the flow rate through the unit. On our water system at full force the water comes out pretty hot in the summer when the water temp in the tank is quite high but needs to be shut down in the late fall early spring when the water temperature is colder. On any boat water system it should work fine as you pressure is not as high as on land.
It turns the burner on and off as you open and close the valve so it is very economic on the propane.
The ignition is with a a push button pizeo button like you see an many house gas hot water heaters. You slide the button into light position, press the button and hold it then a thermocouple sets up so you can let go and slide into on position. the pilot light then remains on until you slide the button back to off. So you may light it for a period when all might use hot water then turn it totally off and shut the shutoff so no propane is on or can be turned on.