Richelieu Rock in October

November through April offers the best conditions at Richelieu Rock, with calm seas and 25-30 meter visibility. December and January bring the highest chance of whale shark encounters — water temperatures drop slightly, bringing nutrient-rich currents that attract the big pelagics. March and April can get crowded as dive boats from around Southeast Asia converge here before monsoon season starts. February hits the sweet spot with good visibility, manageable crowds, and active marine life. Avoid May through October completely — monsoon swells make the crossing dangerous and visibility drops to 5-10 meters. Water temperature stays around 28-29°C year-round, but bring a 3mm wetsuit for multiple dives per day. Early morning dives often produce the best whale shark sightings, so book liveaboards that hit Richelieu Rock first thing.