March – October, marks the entire diving season within Koh Tao.
July – September are considered the best months to dive in Koh Tao as during this time water visibility is at its greatest and waters are calm.
July – August = Tourist High Season in Koh Tao. This is when the most other divers will be present.
March – May = the best months to dive with Whale Sharks in Koh Tao. This is due to the large plankton blooms that occur in the water at this time, attracting Whale Sharks and other pelagics such as Manta Rays. This does reduce water visibility somewhat.
November – February is monsoon period in the Gulf of Thailand. With rain and choppy seas, water visibility is reduced and turbulent waters make conditions less than ideal for beginners.
Technically, good diving can still be had during the monsoon period in Koh Tao, however water conditions are less consistent and harder to predict, making this hit and miss.
At this time of year, it is better to dive in locations in Thailand’s Andaman sea such as Koh Phi Phi, Koh Lipe and Koh Lanta.