Jim - If you have reverse cycle air conditioning then you have a good source of heat until the water gets down to 38 degrees. Of course this system needs a lot of power so you'd need a genset to use it away from the dock. We installed a diesel heater from Espar and used it on the Chesapeake through four winters. Since we lived aboard we ran it 24/7 from early December through early March and found it to be very reliable. It does require maintenance but the unit is very serviceable and we found the local dealer and the factory reps to be excellent.
Installation requires some serious planning ahead but the manual has a number game formula to help lay it out that actually works. Once it's installed the most important regular maintenance is keeping the fuel system clean. It's suggested to run the system once every month throughout the year to prevent the fuel pump from going bad and we found this pump to be the biggest problem. The fuel injector and atomizer screen require occasional cleaning or replacement but service is pretty easy.
Here's a link to our heater installation and troubleshooting (
ip42035-9.blogspot.com). Most recent post is at the top so look further down the page for the installation section.
Tom Ward
IP420-35
La Vida Dulce