Koh Samui has its own climate cycle, out of step with much of Thailand. The driest, sunniest months run roughly from December to April, with January and February the most settled. June to September is also largely dry and a good alternative when the Andaman coast around Phuket is wet. The island's own rainy season peaks in October and November, when the north-east monsoon brings the heaviest rain, rough seas and occasional closures of marine-park tours.
Scooter accidents are the single biggest risk for visitors, on hilly, fast and sometimes wet roads. Wear a helmet, carry a valid licence, avoid riding after drinking, and take extra care at night and in the rain. Check that your travel insurance covers motorbike riding.
The sea is usually calm and warm, but surf, currents and jellyfish can appear, especially during the October-to-December monsoon. Watch for warning flags, ask locally before swimming far out, and supervise children closely on the busier beaches.
October and November bring the heaviest rain and rough seas, which can cancel boat trips to Ang Thong and Koh Tao and make roads slippery. If you travel then, keep plans flexible, allow buffer time before onward flights, and expect some tours to pause.