Snorkeling the Great Barrier Reef is generally very safe, provided you follow standard safety protocols and use a reputable tour operator. Operators provide life jackets or "noodles" for buoyancy, and professional lookouts monitor the water at all times. The biggest risks are actually environmental, such as sunburn or dehydration, and biological, such as "stinger" (jellyfish) season from November to May, during which time guests are required to wear full-body Lycra suits. While reef sharks are common, they are typically harmless to humans. The most important safety rule is to never snorkel alone and to be honest about your swimming ability; most incidents occur due to pre-existing medical conditions or exhaustion rather than marine life. As long as you stay within the designated areas and listen to the safety briefings, it is a world-class, low-risk experience.