Sailing all year long!
The nice thing about Thailand is that you can sail all year round !
The average temperature is 32°C, so no need for a raincoat ! And the average temperature of the sea is 28°C, which should meet the needs of the most sensitives to cold !
The Andaman Sea is not very agitated and generally not very dangerous. In Thailand you can mostly visually navigate but you have to be careful about reef, tide et current.