Seriously Jon, but I also think it's likely the damage is already done January 3/4th when it went down to 20. Also your engine is the biggest concern. Cracked blocks do happen in Norfolk (I surveyed a boat I didn't buy because of that).

What you can expect is cracked faucets and any other valve that's closed and traps water inside (e.g. ball valves), and pumps/expansion tanks. Also, any fittings where water exits a tank into a hose. Se cocks are not really an issue because they're underwater, and the extreme temps are transient.