What the flags mean
The system is simple. Red and yellow marks a lifeguarded zone and is the safest place to swim. A single red flag means no swimming. Two red flags mean the water is closed entirely. Yellow means difficult conditions, take care. Black and white chequered marks a craft area for boards and kayaks, not for swimming.
The key difference from European resorts: a red flag in Phuket is almost never about wave height as such. It goes up because of rip currents, which are invisible from the beach, and the water can look perfectly calm at the time. That is why the ban is so often ignored — and why tourists drown here every year.
Since 2019 swimming under a red flag on Phuket beaches has been formally prohibited and carries a fine. Fines are rarely issued in practice, but lifeguards will insist you leave the water, and arguing is pointless: they can read the current in the wave pattern and you cannot.
Rip currents: the island's main hazard
A rip forms when water pushed ashore by waves returns to sea as a narrow fast channel. Such a flow can reach two metres per second — faster than most people swim. It does not pull you under, it carries you away from the beach, and what actually kills is panic and exhaustion rather than the current itself.
How to spot one: a strip where waves do not break but seem to collapse; water murkier and darker than its neighbours from stirred-up sand; a visible stream of foam and debris heading out from the shore. If one stretch looks suspiciously calm against the surf around it, that is probably it.
If you are caught: do not swim straight back against the flow, you will not win. Stay afloat, raise an arm, get attention. Swim along the beach, parallel to the shore — rips are narrow, typically ten to thirty metres, and exiting sideways is realistic. Only once you are clear, angle back to the beach.
Higher-risk season is May to October, the southwest monsoon, on the west coast: Patong, Karon, Kata, Nai Harn, Kamala, Surin. Rips occur in the dry season too, just markedly less often.
Jellyfish
The Andaman Sea has Portuguese man o' war and, far more rarely, box jellyfish. Mass arrivals are unusual in Phuket: wind brings them in episodically, more often in the wet season, and beach services post warnings on those days.
First aid for a sting: get out of the water and douse the area generously with vinegar — many beaches have posts with bottles for exactly this. Do not rub it, do not rinse with fresh water and do not pour alcohol on it: all of these make remaining stinging cells fire. Lift clinging tentacles with tweezers or a card edge, never bare hands.
If breathing becomes difficult, or there is severe pain, weakness or nausea, call for help. Thailand's emergency number is 1669 and the tourist police is 1155. Box jellyfish reactions develop fast, and this is not a case for waiting it out.
Tides and things nobody warns you about
Tides in Phuket are pronounced: on shallow beaches like Mai Khao or Bang Tao the water retreats tens of metres at low tide. On sea tours this shapes the programme — in some bays a boat simply cannot reach the shore at low water and the landing is moved.
Sea urchins sit among the rocks at the edges of bays. Reef shoes solve the problem completely and cost around two hundred baht on the island.
A small but recurring story: at Phi Phi and Coral Island in high season people are injured by longtail propellers after swimming past the buoys. The buoy line is not advisory — boats run beyond it.
And the general rule that covers nearly everything: do not swim alone and do not enter the water after drinking. In the island rescue services' own statistics these are the two leading contributing factors across all incidents.