If you are diving, it depends on how cold your body naturally gets. I've seen a dive guide wear two layers - a rash guard under a wetsuit - when the water temperature was in the upper 70s. On that same dive, another diver was just wearing a rash guard and swim trunks. You have to make sure you are going to be comfortable the entire dive so you don't have to call the dive earlier than you would like because of hypothermia. It is easier to take layers off, than it is to put on an item of exposure gear that you don't have with you. Alternatively, you don't want to put on too much on the boat too early, or you will subject yourself to overheating and possibly getting sick on the boat. You might want to touch base with the shop you are diving with in the Caymans to see what they recommend for the time of year you are going. That's always helped me to know what to bring.