the best option is the catalytic buddy heater, the single cylinder model.
I was reluctant to get one because of the moisture issues I have experienced for many years with cooking and using propane in a travel trailer.
but the good news is, in practice, using the heater in a small boat isnt as bad as in a travel trailer because, 1, even though a byproduct of burning fossil fuels is water vapor, you are already on the water where there is a higher amount of moisture in the air, and being in that environment does NOT create any more or less byproduct from the combustion, but lessens its noticeable effects.... 2, usually, with the temps well above the freezing mark, you arent running it as long in that environment, and 3, the boat is not closed up as long as a travel trailer commonly is and without as large of an interior, when the hatch is opened, the hot damp air can escape quicker, and lets in drier air....
I was surprised at how nicely the buddy heater works.... if it wasnt for my wife needing a reliable source of heat in the cold mornings, I would still be holding firm with the naysayers....
I do agree that a CO detector is good practice, no matter if you have a heater or not, but where the buddy heater is a catalytic heater, the claims made by the manufacture is that it can safely be used indoors... but I would think one needs to then monitor the O2 levels so you dont consume all the oxygen out of the space....
we like ours and will continue to use it as needed....
there are no older outboard motors that create enough heat while idling to efficiently heat a living space. they are set to run at less than 140 degrees.
cold weather fishermen do commonly tap into the water system of their outboard and run a stream of water into a tank for washing and warming their hands, but even when a large outboard motor is working hard, the water is never hot enough that you cant hold your hand in it.....
this may not be true for some of the newer 4strokes and etec motors, but still, a 2 cylinder motor at an idle could not produce enough heat or water flow to heat a 200+ cf living space.