I lived on a boat for 15 years in cool San Francisco, which pales in comparison to winter in Boston. But I'll second the comment that moisture is the enemy. Every breath you take adds water to the air, and if that water is not removed, it will condense on the cold hull and cause no ends of problems.
The only way to get keep condensation from forming is to remove the water. Either with a dehumidifier or through ventilation--or both. The problem is the colder it gets, the less ventilation you want because it is... well... cold!
I had a in-cabin heater, a Dickenson diesel fuel model that was great at keeping things warm and dry on a 40 foot boat down to 20 degrees or so. It would do the main saloon on a SM, but not the whole boat in Boston temperatures. It's also a major installation project.
Some other simple hints...