If you want to get elaborate you could use automobile or truck fan heaters. You can get them cheaply at an auto wreckers. You need to run insulated pipes in a loop circuit with the radiatirs in the loop in parallel so closing one off does not close off the other. The master heater is either an external gas or
electric outside the
boat cabin. A small
electric impellor keeps the
water moving. When the engine is running the
coolant can also be run through the heaters if the vessel has a
heat exchanger and not
raw water cooling, before going through the
heat exchanger. The safest system there is--no open flames, no heavy
current electric faults possible, and a
cheap electric hot
water storeage
service will do the job if you use
power. They draw about five to eight amps when on--and will be on about ten percent of the time. If you wish to use gas, a more elaborate small storeage system is needed but it is still do-able with all the gas outside. The
battery drain for the impellor
pump is quite low--it is only necessary to move the water slowly.