It's been about 40 years since I spent a winter on board a sailboat in the NE winter, it did hit 27deg in St. Augustine last January. Steel
is not a good idea because you will be sitting in 36deg water
with temps in the low 20s. If you are in a slip then electric
heaters should keep you warm and dry. The only thing worse than frost on the inside of a tent is frost on the inside of your boat. If you are on a mooring
then plan on plenty of propane
but it probably won't matter because the dingy ride will kill you first. Most of your heat loss will be verticle through your deck
and any openings. rather than pick a boat for how well it will get you through the winter I would choose one for how well it does in the summer, that's when the perfomance matters.