Samui (Gulf side) gets its main rain from the northeast monsoon, October to January, wettest around November. The dry, hot season runs February to May. The southwest monsoon (June-September) is mild on Samui.
Koh Samui watch
Samui's calendar is the reverse of Phuket's and most of Thailand's, because it sits on the Gulf of Thailand side, sheltered from the southwest monsoon:
October to January, northeast monsoon: the real rainy season. The ERA5 1991-2020 normal at Samui gives about 282 mm in October, 284 mm in November and 148 mm in December: close to half of the roughly 1,530 mm year falls in October-December. Floods, rough seas in the Gulf and the rare cyclones come in this period. It is also when reservoirs refill.
February to May, dry and hot season: February is the driest month (about 39 mm). March to May is the heat peak. Water stored during the monsoon is used up.
June to September, southwest monsoon: Samui is relatively sheltered; rain is moderate. Andaman-side storms and wave warnings mostly do not apply.
The TMD monthly summaries describe the same pattern: heavy to very heavy rain on the east coast of the South in November-December under the northeast monsoon.
Why it matters for Koh Samui
With El Nino, the danger window for water is February-April 2027, and for heat March-May. The flood and sea risk is highest now, October-January, El Nino or not.