We leave our boat in the water all winter and have a BoatSafe heater in the engine room that keeps things about 40* down there, and several 1500W heaters throughout the interior of the boat.
I goofed up a couple of years ago and forgot about a sea strainer in the lazarette. It was an unheated area. The temps dropped, the plastic bowl in the strainer froze and broke and I ended up having to replace it.
Now I make sure I drain that strainer and also had a 110V plug installed so I can plug in a small 750W heater.
Mike and Tina
Beachcomber 1995 Sea Ray 550 Sedan Bridge