Marine Stingers. Two types of marine stingers, or jellyfish, are found on Cairns' Great Barrier Reef. Both are poisonous and can leave painful stings and sometimes scars. Usually, they are only present in the waters between November and May, but this may change pending weather conditions.

Snorkeling in Cairns is generally very safe, but it requires awareness of the tropical environment's specific risks. Most visitors take commercial tours to the Great Barrier Reef, where professional lifeguards and "lookouts" monitor swimmers from the boat or pontoon. The primary safety concerns are marine stingers (jellyfish) and the sun. Between October and May, "stinger season" is in effect, and it is mandatory to wear a full-body lycra or neoprene "stinger suit" to protect against Irukandji and Box Jellyfish. While these jellyfish are more common near the mainland, they can occasionally be found on the reef. Sunburn is another major risk; the UV levels in North Queensland are extremely high, so waterproof sunscreen and protective clothing are essential. For those who are not strong swimmers, tour operators provide buoyancy vests and "noodles" to help them float effortlessly. Additionally, visitors are briefed on the importance of not touching the coral, both for their own safety (some corals and fish can sting or scrape) and to protect the fragile marine ecosystem. As long as you follow the instructions of your guides and stay within the designated areas, snorkeling is a low-risk, high-reward activity.