Koh Tao was formed through land subsidence of the surrounding area, which is now the sea floor, making the depth of the sea around Koh Tao and Koh Nang Yuan at the most 30–40 metres. Koh Tao has a diurnal tide meaning there is only one high and one low water each tidal day.
Conditions are changeable throughout the year, with Koh Tao’s monsoon usually arriving between the end of November and beginning of December. Heavy rain, strong winds and waves are normal at this time, though it only lasts a few weeks. Boat services usually continue even with waves up to 4 metres high, but they can be interrupted if the conditions are too bad.
From October through to March, the tide is high, so it’s better and easier in places for swimming and snorkelling with the beaches and bays full of water. From April through to September, the tide is low during the day making the beaches wider for sunbathing and beach activities.