NavList:

I think the variation in refraction is a good point.  In fact, my first experiments were last fall, and at least in the upper Bay near me, mirages are quite common in the fall (due presumably to the air-water temp. difference) and it was nearly impossible to figure out where exactly the distant shoreline was -- since boats were sometimes seen sailing above it!  I suspect that such effects are equally common at sea but might go unrecognized because there are no background objects that would reveal the problem.

One of my motivations in writing this tech note is to encourage celestial nav training (or at least experimentation) at the Naval Academy; I know an instructor there and sent it to her.