Get a butane hotpot stove; just dont leave the canisters on your boat. Incredibly hot, incredibly cheap. It's by and far the cheapest and easier solution.

The stoves are around 30$, and the canisters are about 2$ each.

https://www.amazon.com/Portable-Butane-Stove-Carrying-Listed/dp/B001TF8UY8/ref=sr_1_10?s=outdoor-recreation&ie=UTF8&qid=1490967170&sr=1-10&keywords=butane+stove