I know this is an old thread, but thought I would put my 2 cents worth in just in case others have concerns about the sun. If you are not necessarily wanting to get a tan, and just wanting to snorkel or swim, particularly in the ocean, I would recommend wearing a full lenght dive skin. These garments are a lightweight nylon/lycra blend and cover you from neck to ankles. (Not the same as a neoprene wet suit). They provide excellent protection from the sun AND jellyfish stings and are not too hot to wear. I used one on a recent trip to Hawaii and loved it.
Just remember to use sunscreen on your face.