I used to race triathlons, and I've dived into my fair share of chilly lakes and rivers at 6 AM. I guess this is a different thing than a leisurely swim, but t

I used to race triathlons, and I've dived into my fair share of chilly lakes and rivers at 6 AM. I guess this is a different thing than a leisurely swim, but the main thing I remember is the pain in the temples and around the crown of the scalp from submerging the head in ice-cold water. Something like a brain-freeze from eating ice cream too fast.