did you have an ice-jam on the exterior?
When the refrigerator shuts down, so does the water pump.
Depending on where yours discharges, the line to that point, may not be freeze protected by boat interior heat.
Mine ran under by cockpit and above the marina water level on the hull. Therefore
totally exposed to outside temperatures. Even thought it may be salt water, it can still freeze.
In addition to Gunni's point above on the refrigerant heat exchanger, in extreme cold, the water can freeze on exit.
If you are a live aboard, then your refrig would just throw a breaker, but if not, you can
bust a water line.
If you are NOT there and the
busted line thaws, the refrigerator water pump can start again and...
SINK your Boat!!!

Jim...