The dry season runs from November to April, when the Andaman Sea calms and Surin Beach is at its best for snorkelling and swimming. December and January are the peak months — busiest and most expensive, but reliably beautiful. May to October is monsoon season: waves build, the undertow strengthens, and red flags go up regularly. The beach is not swimming-friendly in the wet season, though surfing picks up and the dramatic skies and emptier beach have their own appeal.
Surin is busy with Thai domestic visitors on weekends and Thai public holidays year-round — if you want a quiet beach morning, weekday visits in the dry season are ideal.