[Bill S (OP)]

There are plenty of books, Nat. Hist. Progs. etc that explain how polar bears, and other animals, survive in the sea, but how do their eyes cope with such cold water?

[Don_1]

I'm none too sure about this, but I think antifreeze proteins might have something to do with it.

Perhaps Oceans Helen (Helen Scales) might be better informed to answer this question.

[chris]

I think it's fair to say that although the water is cold, the eye has a rich blood supply and so remains sufficiently warm so as to function efectively.

Chris

[Oceans Helen]

I'm not aware of any polar animals - including polar bears - that have antifreeze in their eyes (but nice idea!). and I agree with Chris that probably their blood supply would help keep them thawed out.

Seawater can drop to minus 2 degrees C, which is pretty cold, but temperatures get far lower out of water with cold air plus wind chill factor - so really, any polar animals (including human visitors) risk getting much nipper out of the sea than in.

[SteveFish]

Antifreeze proteins were discovered in a large number of related arctic fish species. As Chris said, polar bears, which are just a variant of brown bears (they can interbreed), have good blood circulation, and the eyes are near the brain which is the last place where warmth and oxygen are conserved under extreme conditions.