Great Barrier Reef Marine Park in June

April through October gives you the sweet spot — dry season with calm seas and excellent visibility. May and June are perfect: water temperature sits around 24°C, crowds are lighter, and coral spawning hasn't stirred up the water yet. July through September brings the busiest crowds but also the most reliable weather. Stinger season runs October through May, when box jellyfish and Irukandji make swimming dangerous without protective suits. Most operators provide stinger suits during this period. November through March means stinger season, cyclone risk, and heavy rains that reduce visibility. But here's the upside: accommodation prices drop 40%, and you might have entire reef sections to yourself. Water temperature peaks at 29°C in summer — perfect for extended snorkeling sessions if you can handle the stinger suits.