One of the boats I've used has a Webasto blower and a Wallas two ring diesel hob. I can't speak for the Webasto, but have found that the hob puts out enough heat to warm the cabin (5'x6') appreciably. Also I believe that Wallas do a blower unit that fits onto the hob, so you could end up with a heater/blower and a hob all in one.
Finally, from my college days, the cheapest way I've found to heat a small space is to fill it with bods, have a natter and a cuppa and wait for the weather to warm up, dry up, etc.
Regards,
Lockie.