I used to have a rolled up bandana that would form a tube when soaked in cold water, and you would tie it around your neck. The cold part would rest on the back of your neck and keep you cool. I used it a lot while I was hauling hay and it helped, but it was a pain to tie on, and it would slip around and get coated with dirt. Haven't ever thought of finding one to use while fishing.
Vests and caps sound like they would work better....I wonder how heavy they are?