I have had various heat sources in boats. My current Boat has Propane heat which is OK and is as others have mentioned pretty instant. I find it doesn't really take the chill out though as well as my old wood in the last boat. My last boat had a great little wood stove
built in the lunenburg foundry in NS. I loved it. It heated the boat wonderfully, took the chill out, and burned little fuel
. We could get the fire going with small kindling and then throw on Charcoal and it would stay toasty all night (it was very airtight.) It was also great on a rainy afternoon or evening with the kids. We could open the grate up and have a marshmellow roast.