if you have 12 or 24 volt dc, you can use the very effective dc heaters available. I use one on an open 24 foot boat in puget sound. Just the thing for cold hands. They can be very effective. I have some batteries for my Torquedo 2hp electric outboard which will drive my umiak tri at 11.1 knots.
The Eskimos heat and seal the inside of their Igloos with a single lamp made from wick and blubber so what ever heat you'll need it'll not be much. As stated by others naked flames are a no no, gas and liquid fuels will just add to the condensation.
In a boat you'll always be looking for a dry heat. For curing of resin and the drying of boats I always use either infra red or halogen heaters. Even a 500 watt halogen light gives off a fair blast of heat, but as to how you can run a halogen heater off of a 12v system would take a bit of working out.
If all else fails look into getting a small nuclear reactor!
Check once the Webasto Air Top 2000 ST Dieselheater. May be you find a way to install the unit external to the small cabin and lead the warm air through a insulated pipe to the cabin. How ever you will need a considerable and continuous electric supply and this means heavy batteries and this for a rather small boat?
Most simple way for heating temporarly a small cabin is a simple petrol storm lamp and a well working ventilation or use your camping gas stove and heat over the flam up upside down old fashioned flowerpots made of clay, this is very efficient. Heating and ventilation on small boats is in general a matter of fuel-/energy capacity. Cheers Wave PS: Of course best way is to avoid cold latitudes!
I found this possible solution to the latest request in this thread. I'm sure a small but fairly well insulated pod could be installed just in front of the main beam to duct the air flow into the cabin, over the sleeping sweaty bodies and exit through the main hatch, suitably screened to prevent the entry of insects.