"Small boats are not noted for their insulation."
This is true , but easily over come ., MORE Heat.
In NYC the usual winter is mid 20 at night , just freezing most days.
Our 90/90 has a Dickinson floor mounted oil burner that is about 20,000 BTU.
This worked fine with an Airex foam core hull, and extra rugs on the cabin sole .
Never bothered to add insulation storm covers or warm shades to the dozen bronze ports , although they would steam , then frost for a while on dumping a pot of hot water in the sink.
There is a wall mounted fire place in the fwd cabin , but it usually got fed a piece of fire log , for dates , although charcoal could be added for overnight heat.
Mostly used in fall when Dickinson was not lit and an overnight of cabin heat was required.
For me the key to living aboard in winter was the lack of electric required to maintain the std of living.
It does require advanced planning and some bucks to set up for real living aboard , but all the hassles are well known and fairly easy to overcome.
A couple lived aboard on a 41 Bounty , one of the early GRP boats with inch thik layup!
No insulation but an oil fired range kept then warm , except at a record -17 F when they had to abandon the fewd cabin and sleep in the salon.
It can be done and is a great lifestyle!!