once you get acclimatized to the local waters your core body temperature tolerance adjusts to it, thats why drysuits and underwear are used in Mexico and Florida in the caves. Recent trip to the Keys the locals thought I was nuts diving a 3mm shorty in nov while most of them were diving in 3/5 suits. Do what we do in the great white north, bring a thermos of hot water for the second dive and pour it in your suit.