Whether you need a wetsuit for the Great Barrier Reef depends more on the season and your protection needs than just the water temperature. During the Australian winter (June to August), water temperatures can drop to 22°C (72°F), which feels quite chilly during a long snorkel session; a 3mm shorty wetsuit is highly recommended for warmth. In the summer (December to February), the water is a balmy 28°C (82°F), and you might not need a wetsuit for warmth, but you must wear a "stinger suit" (a thin lycra suit). Stinger suits protect you from the potentially lethal Box Jellyfish and Irukandji which are prevalent in the warmer months. Beyond temperature and stingers, wetsuits provide excellent sun protection and act as a physical barrier against sharp coral scrapes. Most reef tour operators include a stinger suit or wetsuit in the price of the trip, so while it isn't a legal "requirement" to wear one, it is a standard safety and comfort practice for almost all visitors to the reef.