I leave my diesel heat on 24x7 this time of year. Besides that, putting on outdoor covers, if you have them, adds a little more insulation to the single pane windows my boat has. I also drop the blinds inside, although I leave about a 2" gap at the bottom so some air can circulate. I find dropping them all the way causes more condensation in the windows because the air is trapped there.
In the lazarette, I have one of those circular fan heat things that West Marine sells to add a little bit of warmth. All of the other bilges are under the main part of the boat and shouldn't get too cold.
For the engine room, I use pipe insulation that you would normally use in a house, and stuff that in the cracks around the vents, closing off the whole engine room. I leave the AC hot water tank on, and the hydronic heater itself plus that generate enough heat to keep the engine room at a toasty 70F plus.
We got about 3" of snow at Elliott Bay Marina over the day yesterday. The wind was gusting beyond 20 knots, and blowing snow everywhere for most of the day. Today looks pretty calm and cold.
The low last night, actually this morning at 7AM, was 23F and a low wind chill of 17.2F
Hydronic is keeping things at a nice 60F while I am away from the boat!