Derawan Islands in December

April through September offers the calmest seas and clearest visibility for diving. Visibility regularly exceeds 30 meters during these months, and boat rides between islands are comfortable. This is peak season, so book accommodations well in advance. Manta ray sightings peak from May to August when plankton blooms attract them to cleaning stations. Whale sharks are present year-round but seem more active during the calmer months. Avoid October through March when northeast monsoons bring rough seas and frequent rain. Many dive operators close during January and February due to dangerous conditions. Even if you can get there, you'll spend most days stuck indoors. The shoulder months of April and September offer the best value with good weather but fewer crowds. Water temperature stays around 28°C year-round, so a 3mm wetsuit is plenty.