Sounds like you have raynaud's syndrome, as I do. Basically the capillaries in your fingers shut down. The pool doesn't usually set my hands off (I refer to it as my fingers 'going away' and 'coming back'), but there's really no rhyme nor reason as to what's going to cause it for me on a particular day. I usually use hot water to bring them back.

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Any run that doesn't include pooping in someone's front yard is a win.

It could be thoracic outlet syndrome. It commonly affects overhead athletes, particularly if technique isn't exemplary and/or upper body musculature has an anterior/posterior long/weak-short/strong mismatch.

http://orthoinfo.aaos.org/topic.cfm?topic=a00336

Does it happen during/after biking or other activities?

Agree it does sound like Raynaud's. Strange that a warm pool would set it off though. I get it in my hands (but not my feet) when running in the winter, and in my feet (but not my hands) when biking. I actually like swimming in the cold months (outside pool) just because I don't have to deal with the Raynaud's!