First, as I recall, ice expands to about a little less that 10% in volume from it's liquid state. So if "restrained' by something that can't take a stress or is strained beyond it's elastic limit, something is going to give. A plastic hose of a sufficient diameter is propable okay, Some fitting may or may not be, etc.;

Secondly, the pressure (usually in a domestic system) that remains in the line, tends to exacerbate the issues. While you like to have the heat sink effect of mitigating the freezing tendancy by allowing the water in the system to get some heat from the "inside' of the boat (and not freeze), if you do get blockages with a pump or other pressure, you can increase the pressure inside the pipe.

Anyway, who knows in Jon's case. Having 20 degrees outside temperature with 40 degree water in NO wind would cool down the boat less than having the same with more cooling on the side if the hull in a wind. (remember, direction of the wind, actual circulation around the hull, etc. make a difference).

As Jon said, one night of 20 degree as a low -- with warmer water and a "heat-up" during the day is a lot different.