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.