bobgads, it depends on what part of the country you are visiting. We have two monsoon periods here, one when the prevailing winds are from the northeast and one when the prevailing winds are from the southwest. Right now we are in the southwest monsoon period, so the winds are kicking up the waters of the Andaman Sea on the west side of the country, and bringing some rain along as well. During this period, the waters of the western Gulf of Thailand are more protected and it rains less. The southwest monsoon is roughly from May to November. When the northeast monsoon is blowing, it's basically the reverse--the seas on the Andaman coast are generally flat and skies are blue. The northeast monsoon is roughly from November to May. Most divers who travel from half a world away, like you, plan to come in the winter months, when the northeast monsoon is in force, allowing Thailand's most celebrated dive sites in the Similans and Surins to safely welcome divers.