Can you swim in Thailand in December?

December is one of the better months to be in the water in Thailand, at least on the Andaman coast.

The Andaman coast

Phuket, Krabi, Phi Phi, and Koh Lanta are in full dry season in December. The Andaman Sea in December is calm, clear, and 29-30°C. West coast Phuket beaches (Patong, Karon, Kata, Surin, Kamala) all have green swim flags. The water is flat enough for children and non-swimmers to wade freely. Snorkelling in December is straightforward: clear water, good visibility in shallow reef areas.

Dive conditions are excellent. Visibility builds toward 20-25m as the dry season establishes. The Similan Islands are open and running full schedules from mid-November onward. For diving specifically, December is in the good window that runs through April.

The Gulf coast

Koh Samui and Koh Phangan are transitioning out of their October-November rainy season. The first half of December can still see northeast monsoon swells on the Gulf side: some rough days, occasional wind, and less predictable conditions. Swim flags can be yellow (caution) during these periods.

By mid-December, the Gulf usually clears. Koh Samui's east coast beaches become swimmable with calm conditions. Koh Tao is more sheltered than Samui from the northeast monsoon and is typically swimmable from early December on its west side.

If swimming is the primary reason for the trip and you're on the Gulf side, choose the second half of December to be safe.

Water temperature

Both coasts in December have sea temperatures of 28-31°C. You don't need a wetsuit for leisure swimming anywhere in Thailand in December. Divers doing liveaboards at depth sometimes wear a 3mm shorty but it's a comfort choice, not a necessity.

The flag system

On any Thai beach, the flag system tells you what's safe. Green flag: swimming is safe. Yellow flag: caution, strong swimmers only. Red flag: no swimming, usually riptides or dangerous surf. In December on the Andaman coast, flags are almost always green. On the Gulf coast early in the month, occasional yellow or red flags appear during northeast wind events.

Whale sharks on Koh Tao

December is the start of the whale shark season at Koh Tao, which runs roughly November through February. Whale sharks come to feed near Chumphon Pinnacle and Sail Rock. Seeing one isn't guaranteed, but December through January is the window when encounters are most reported.

For the full December picture, see our Thailand in December guide.