The dry season runs from November through April, and this is when the bay is at its calmest and most photogenic — clear water, reliable sunshine, and manageable winds. December to February brings the most visitors, and the main sights (particularly James Bond Island) can feel crowded by mid-morning.
The monsoon season from May through October brings rougher weather, but the bay does not close entirely. Some days are fine; others are not. Boat tours may be cancelled at short notice when swells pick up. The upside is dramatically fewer tourists and lower prices. The rocks and mangroves look particularly lush in the wet season, and the overcast light can produce striking photographs. If you are planning to sea kayak into the hongs, go with a guide who knows which caves are safe at which tides regardless of the season.