Koh Tao diving by month: when to come and what you'll actually get
Written by the instructors at Carabao Diving Resort, Chalok Bay — we dive these sites every day of the year, so this is the calendar we give our own friends.
Short answer: the most reliable months are February to April (flat seas, 15–30 m visibility, whale shark season). The insider pick is September to early October — similar visibility, half the divers. The only month we'd tell you to avoid is November, the heart of the monsoon. Water is 27–30 °C every single month, so a thick wetsuit is never needed.
Koh Tao sits in the Gulf of Thailand, which matters more than most visitors realise: the island is sheltered from the southwest monsoon that hammers Phuket and the Andaman coast from May to October. That's why our "low season" is short and our diving year is long. Here's every month, honestly.
January
Visibility 10–25 m · Water 27–28 °C · Crowds: high
The tail of the northeast monsoon. Early January can still carry some wind and chop from December's weather, but conditions improve week by week. Sheltered sites like Twins and White Rock dive beautifully; by late January the whole island is usually open. Peak tourist season, so book courses and rooms ahead.
February
Visibility 15–30 m · Water 28 °C · Crowds: high
One of the best months of the year. Seas flatten out, visibility stretches, and every site on the map is diveable. If you want textbook Koh Tao — glassy morning boat rides, blue water at Chumphon Pinnacle — February delivers it.
March
Visibility 15–30 m · Water 28–29 °C · Crowds: high
February's conditions continue, and the first serious whale shark window opens. Sightings pick up at Chumphon Pinnacle and Sail Rock through the month. Great month for the Open Water course: calm, clear, warm.
April
Visibility 15–30 m · Water 29–30 °C · Crowds: high
The hottest month on land and prime whale shark season underwater. Mid-April brings Songkran, the Thai New Year water festival — the island parties, and boats fill up fast around the holiday. The sea itself is usually calm and clear.
May
Visibility 12–25 m · Water 29–30 °C · Crowds: medium
Whale sharks are still around, the peak-season crowds are not. Occasional afternoon showers begin, but they rarely touch morning diving. One of the smartest value months on the calendar.
June
Visibility 12–25 m · Water 29 °C · Crowds: medium
Settled, warm, easy. The Gulf stays calm while the Andaman side gets rough — June is when Koh Tao's sheltered position starts paying off. European summer travellers begin arriving late in the month.
July
Visibility 10–25 m · Water 29 °C · Crowds: high
Summer holiday season. Conditions are generally good with the occasional windy spell; visibility can swing but often surprises on the high side. Book your course before you arrive — schools with small groups fill up in July.
August
Visibility 10–25 m · Water 29 °C · Crowds: highest
The busiest month of the year thanks to European holidays, and the sea mostly cooperates. While Phuket cancels boats, Koh Tao keeps diving — this is the Gulf-side advantage in action. If you're coming in August, reserve diving and accommodation well ahead.
September
Visibility 15–30 m · Water 29 °C · Crowds: low–medium
Our honest favourite. The crowds leave, and the water often turns spectacular — some of the year's best visibility happens in September. Boats are quieter, dive sites feel private, prices soften. If your dates are flexible, come now.
October
Visibility 10–25 m · Water 28–29 °C · Crowds: low
The second whale shark window, and a genuinely underrated month — early October often continues September's conditions. Late October starts the transition toward monsoon: a little more wind, a little more rain, day by day less predictable.
November
Visibility 5–15 m · Water 28 °C · Crowds: lowest
The monsoon month, and the only one we'd steer you away from. Heavy rain, rough seas, reduced visibility; boat trips run when conditions allow, not on a fixed schedule. It's still diveable on many days — but if you're flying across the world for this, pick another month.
December
Visibility 10–20 m · Water 27–28 °C · Crowds: rising to high
Early December can still be unsettled as the monsoon exits. From mid-December the island switches on: weather stabilises, high season begins, and the Christmas–New Year weeks are among the busiest of the year. Book far ahead for the holidays.
Whale sharks: the honest version
Everyone asks. The two strongest windows are March–May and September–October, usually at Chumphon Pinnacle and Sail Rock. But whale sharks read no calendars — we've had sightings in every month, and we've had "peak season" weeks without one. Any shop that guarantees a whale shark is selling you marketing, not diving.
Frequently asked questions
- What's the best month overall?
- February to April for reliability; September for the quality-to-crowds ratio.
- Can I dive during the rainy season?
- Mostly yes. Outside November, rain on Koh Tao tends to be short and doesn't stop diving — the Gulf position shelters the island from the worst of the southwest monsoon.
- Do I need a wetsuit?
- Water is 27–30 °C year-round. A 3 mm shorty or a rash guard is plenty for most divers.
- When should I book ahead?
- December–April and July–August. Small-group schools fill up weeks in advance in these windows.
- Is the course cheaper in low season?
- Course prices stay stable year-round, but accommodation and flights cost less May–June and September–October — the total trip is cheaper.
Dive & stay in one place
SSI Open Water course + beachfront accommodation on Chalok Bay from ฿9,900 — walk from your room to the boat. Small groups, instruction in English, Italian, Spanish and French.
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