My rule is double digits. I think at that point it becomes a safety issue. Below 20 you get these cool sheets of ice that form on your life jacket, helmet, paddle, skirt, boat, etc.
The secret is jog for 10 minutes to the put in. By revving up the engine, you open up your circulatory system. That and mittens. Mmmmmm Mittens.
friend of the fork, knife, and spoon