When you are snorkeling in tropical regions, you should really cover up. Even though the water feels cool to your skin, the powerful rays of the sun can get to you through the water. You could get a pretty bad sunburn with no protection. You could wear a t-shirt, except that would provide you with absolutely no thermal protection. You may want to consider a skintight Lycra suit which will keep you warm and provide protection from the sun. Gloves will also help your hands stay warm and protected.