June-August: peak dry season. Cooler water (22-24°C) means a wetsuit is comfortable. Whale migration in full swing - dwarf minkes off Ribbon Reefs in June-July.
September-November: warmest water (25-27°C), best visibility, coral spawning event in late November.
Stinger season (Nov-May): lycra stinger suits are mandatory on most operators. Outer reef pontoons (60+ km offshore) are safe year-round.
Day trips depart from Cairns, Port Douglas, Airlie Beach (Whitsundays) and Townsville. Booking 2-4 weeks ahead is standard - peak July school holidays sell out.