I have freeze coverage on my boat. Even though it gets winterized, many times water can still be trapped in manifolds water galleys etc. Boat US has this coverage and it is pretty popular in southern states where you would not expect freezing conditions but might happen. Heck latley they are getting as much snow and cold as we are up here. It is an extra $50.00 on my policy. More than makes it worthwhile.
Here is an article from Boat US... back in 2006.
http://www.boatus.com/PressRoom/release.asp?id=150