Why the water goes away from the beach
You arrive at the beach and instead of the sea there’s a hundred metres of wet sand. A few hours later the water is back as if nothing happened. It’s not an anomaly and the sea hasn’t “gone for good” — it’s an ordinary low tide, and in Phuket it runs on a schedule.
Twice a day, following the Moon
The level of the Andaman Sea rises and falls under the pull of the Moon and the Sun. Phuket has semidiurnal tides: the water comes in twice and goes out twice a day, and a full cycle takes about twelve and a half hours. On strong days the difference between high and low water reaches two to three metres. So “when will the water come back” is not a matter of luck but of the tide table: you rarely have to wait more than six hours.
Why low tide is noticeable at some beaches and not at others
Tide times are practically the same all around the island, but how noticeable low tide is depends on the slope of the seabed. On gently sloping beaches with a flat bottom — for example, Bang Tao or Nai Yang — low tide uncovers tens or even hundreds of metres of seabed: the water “goes over the horizon”, and there’s nowhere to swim. At the deeper beaches — Patong, Surin, Nai Harn — the same low tide is barely noticeable: the bottom drops away quickly, and even at low water it stays deep enough to swim.
The two sides of the island behave differently too. The west coast — from Mai Khao to Ya Nui — is open to the Andaman Sea: in the monsoon season the swell comes in here, and on the gently sloping stretches low tide uncovers a wide strip of seabed. Ao Yon on the east side faces a sheltered bay: there are almost no waves all year round, but the bay is shallow, and at a strong low tide you have to walk out to reach swimming depth — it’s more comfortable to swim closer to high tide.
Waves and seasons
The other reason the sea can be unswimmable is waves. From May to October Phuket has the southwest monsoon: the wind blows in from the sea, the swell builds along the whole west coast, and open beaches such as Karon and Kata get rip currents. In high season, from November to April, the sea is usually calm almost all day. Which bays are sheltered from the waves and where it’s calm today — see “Phuket beaches without waves”.