I went to Puerto Rico for a week last year and spent the first several afternoons snorkeling with a minimal amount of sunscreen and no burning to speak of. On the third or fourth day, we were getting ready to leave Buye Beach, and I went down for about 45 minutes of snorkeling before we hit the road to go back to San Juan. Unlike the other snorkel outings, this one was at noon. I left with pink shoulders after 45 minutes. By the time we got to San Juan, my back and shoulders were blistered. I had to wear button-down shirts the rest of the trip because I couldn't move my shoulders enough to get T-shirts on and off.