....having a marine 100w bulb heater to take the edge off, going down once a week and checking on the boat. Even when it's cold air, the water is usually warmer and this helps protect the boat. I've kept low flat heaters running on "freeze turn on" sometimes depending on my weekly visits. 'Bout all I've ever done. Never had anything freeze but never had boats with too fancy appurtenances either.

Marine head, inboard is about all.