Use the same type of Charley Noble you would with a bulkhead mounted propane heater or a diesel fired heater. You can find marine quality stacks and deck fittings here DickinsonMarine.com - Propane Fireplaces
I can't see any type of plastic working - melting, but not working. I have owned tankless propane water heaters in the past and liked the convenience.
If you are surveyed for insurance they will probably ask for its removal as it doesn't meet ABYC standards. Insurance companies hate them. The standards call for a vent, no open pilot light, AND a closed combustion chamber. The only one I know of that can meet the standards is the Precision Temp but it is a lot more expensive.