The dry season runs from November to April, with the best weather between December and February — calm seas, low humidity, and reliable sunshine. This is when most resorts are open and boat connections are at their fullest.
May to October is the southwest monsoon. Rain comes in heavy bursts rather than continuous drizzle, but the seas can get rough and many resorts close from mid-May to mid-October. The dive boats stop running in this period because visibility drops sharply.
If you want to see whale sharks at the dive sites south of the island, March and April are the months to come.