The bulk of the Great Barrier Reef islands lie north of the tropic of Capricorn in a monsoonal climate zone. In the hot, wet season (roughly December–April), expect tropical downpours that can limit outdoor activities and mar underwater visibility.
The warm days, clear skies, and pleasantly cool nights of the dry season, especially June–August, are ideal for traveling around and above Cairns. In the Whitsundays, some winter days may be too cool for swimming and nights can be chilly (pack a sweater and long pants).
The islands are warm, even in winter; during the summer months, temperatures regularly top 35 degrees Celsius (95 degrees Fahrenheit), and the farther north you go, the hotter it gets. The water temperature is mild to cool year-round; however, in jellyfish season (typically, November through May), you'll need to wear a full-body stinger suit to swim anywhere but within patrolled, netted areas.