I'm in the always wear a wetsuit camp and it has nothing to do with scrapes, stings or touching the reef but the older I get the colder I get. Most or my travel is dive related so I'm in the water a lot. My wetsuit gauge is as follows: 50 - 70 deg. full 8mm with hood, 70 - 85 full 5mm and 85+ full 3mm I also bring a 2mm vest just in case. It easier to cool down then to chase the warmth.