I lived aboard in Boston Harbor for years. It can get cold, as you all know, and there are hardships, not the least of which is dealing with the head. I hate it, and for the most part simply used the onshore facilities for all solid waste and only used the holding tank for nighttime liquid.
Since I built my boat, and lived aboard while doing so, I insulated it well, using injected foam as well as the pink and blue sheet foam insulation in the cabin roof and decks. I am also putting this sheeting against the hull all the way to the cabin sole.
Right now, I am in Florida and the insulation helps keep it cool as well as save heat in the winter. I will be going north again soon, as winter is an easier thing to bear than hurricanes stampeding over you.
I NEVER use heat of any kind at night. An accident of any kind will kill you before you can escape, in my humble opinion, and is not worth the risk. I also don't use air conditioning because mold buildup in the unit itself results in headaches.
It might sound like I'm making complaints, but not so: I love this life and cannot imagine going back to land. I have now been living aboard since 1985. It keeps geting better.