Koh Tao is fortunate that thanks to its location and geography you can dive ALL YEAR round with two types of season: weather and tourist.
Conditions are usually favorable from mid-December with temperature and visibility increasing and flat seas until April-May.
May to August can bring changing weather which can include slight showers and windy periods but overall conditions are good. Between the months of September and October conditions re-peak with good visibility and flat seas.
Koh Tao and the Gulf of Thailand has its SE Monsoon usually in November and December with heavy rain at times. Conditions are the worst this time of the year all over the South of Thailand.
The island starts to fill just before Christmas and continues up until the end of April.
There is a second period where the island fills again around about mid-July until the end of September. If you would like to dive during the busier periods we strongly advise that you make a reservation for a bungalow as there is still a shortage of accomodation on Koh Tao. Especially budget rooms are hard to find for late deciders.