Hello, just saw your note today, quite a while since you posted it, so I hope all is well. I've wintered in the water for the last 15 years here in the heartland (Tennessee on the Cumberland River). It has gotten well below zero on occasion, and we've had ice on the harbor. Many of us use the underwater pumps to wash water up around our hull to keep ice from griding away at the finish. If you're in salt, you may not have the freeze problem we do here.
As for internal heat, I keep my heaters on all season if the temp falls much below freezing. I know the bottom of the boat will be above freezing temp if the water is liquid, but the air temp can get into your vents, etc., and can freeze water pumps and exhaust manifolds. Youre using a closed cooling system, so your engines should be generally protected, but the heat exchangers will still be able to freeze at the temp salt water freezes.
I use a digital pyrometer to do quick on board temp readings in cold weather to see how I'm doing. If it looks a little touchy, I'll bump up the thermostat. My marina is next to a public utility water intake, so we have very good electric service. If the power goes off for very long, the boat could be in danger.
I plug all my clamshell vents, etc., with foam sponges to keep the cold air from getting into the boat. Not sure how others manage to weather the cold, but that's how we do it. Every New Years Eve, we always have a bunch of people on board for dinner and cocktails, and it's always a blast.
All the best, Happy New Year,