More facts:

- The boat is trailered.
- It is covered with a standard, trailerable cover; not sure of the
exact material but it is a durable, vinyl type material.
- The small bottle of antifreeze was outside the boat, sitting on the
trailer.
- I used Peak RV/Marine antifreeze, which claims to be burst proof to
-50F.
- The check was done not long after the low temperature.

I realize this situation is hard to diagnose; I appreciate the comments.
I will just have to keep my fingers crossed until I can run the engine
and find out for sure if things are OK or not.

Bob