Cover the boat with trasparent shrinkwrap. Keeps the snow off & creates a greenhouse.
Cover the decks with insulation and cover the insulation with black plastic. Keeps the heat in.
Figure out what you'll do about pump out/waste discharge.
If you get condensation inside...buy a dehumidifier, put a layer of plastic window film over the hatches, open all lockers, run a fan for air circulation, vent your shower to the outside and close off the shower to the inside, avoid boiling water for coffee, tea or pasta...