The solvents used in oil-based finishes will freeze at temperatures far below the freezing point of water--and probably far below any temperature on the surface of the earth--so really cold temperatures won't damage them. On the other hand, the colder it gets the longer it takes for those solvents to evaporate out so the finish hardens, but eventualy it happens.

Letting a varnished boat rest all wilter seems to me like just the right way to do things. After a long season of paddling you pull the boat from the water, sand off the bad spots, patch, sand, and put on a fresh coat of varnish or two. Come spring the varnish is hard as a rock and the boat is ready to go.

PGJ