We have used a Walter instant propane heater on our 35 for about 10 years. It is wonderful - hot showers whenever you want one! I have mine set up so the hot water heater tank (with heat exchanger and electric heater elements) preceeds the propane heater in the hot water line so if the engine heats the water I can use that instead of the propane. The only problem I've encountered is that you have to make sure to drain it carefully in the fall as it is more sensitive to freezing than the rest of the water system.

Marine Exchange in Peabody carries them - a short trip from Portsmouth!
Steve Purdy --C&C 35 - Trader

There are several out there. Bosch makes one as well as Poloma. Poloma says not yo use it in the boat. (Disclaimer)
George Gulden