The sea by month: monsoon, flags, jellyfish and boats.

Two seas, two calendars

The Andaman coast, meaning Phuket, Khao Lak, Krabi and the islands, faces the Indian Ocean and is driven by the southwest monsoon from roughly May to October. In those months the wind blows onshore onto the west facing beaches, the swell builds, the water turns cloudier and rip currents form along the beaches that are calm the rest of the year. June to September is the strongest stretch and September is often the wettest month of all. From November to April the same coast is transformed: offshore or light winds, flat clear water, and the conditions in the photographs. The Gulf coast, meaning Samui, Phangan, Hua Hin and Cha-am, is sheltered from that monsoon and runs later, taking its wet and windy season around October to December, with November usually the worst.

Two practical consequences follow. First, plan sea dependent days on the coast that is in season, not on the coast you have already booked. Second, in the monsoon months build the itinerary so that a cancelled boat day costs nothing: hold the charter loosely, keep a land alternative ready, and never make an island the only reason a family flew.

Reading the flags

The main beaches of Phuket and several other provinces operate a warning flag system, and it is not advisory decoration. A red flag means the water is dangerous and swimming is prohibited, and on Phuket beaches swimming while red flags are flying can attract enforcement and a fine as well as the obvious risk. Two red flags together indicate the beach is closed to swimmers. A yellow flag means conditions are hazardous and swimmers should take care. A red and yellow flag pair marks the area patrolled by lifeguards, and that is where a family should be swimming whenever it is displayed. Some beaches also fly a purple flag for marine stingers.

The instruction we give every client, without softening it: if the red flags are up, nobody enters the water, including strong adult swimmers, including for a short paddle, including if other people are in the sea. Drownings on the Andaman coast in the monsoon months are a real and recurring event, and the pattern is depressingly consistent, involving visitors who judged the water by looking at it. Lifeguard cover is not continuous on every beach or throughout the day, so the absence of a lifeguard is not evidence that the water is safe. Heed the local warnings, and if a hotel, a lifeguard, a boatman or a driver tells you the sea is bad today, that judgement outranks anything in this guide.

Rip currents

Rip currents are the specific danger on the Andaman coast during the monsoon. Water pushed onto the beach by wind and waves returns seaward in a narrow, fast channel, often through a gap in a sandbar, and that channel can pull a swimmer out faster than they can swim back. From the sand they can look like the calmest part of the beach, which is exactly why people walk into them: a patch of flatter, darker, less foamy water with sand or debris streaming out through it. They occur on Patong, Kata, Karon, Kamala, Bangtao and Surin among others, and they can appear on a day that looks perfectly pleasant.

The advice that saves lives is short and every member of the family should know it before they touch the water. Do not fight the current and do not try to swim straight back to the beach against it. Stay afloat, keep calm, and swim parallel to the shore until you are out of the channel, then come in at an angle with the waves. Raise an arm and shout if you can. If somebody else is caught, do not swim out to them without a flotation device; a great many drownings in Thailand are of would be rescuers. Alert a lifeguard, throw something that floats, and keep the person in sight.

Jellyfish and what lives at the waterline

Jellyfish are present in Thai waters on both coasts and appear seasonally rather than constantly, with numbers varying by year, wind and current. Box jellyfish have been recorded in Thai waters and are the serious concern; Portuguese man o' war and other stinging species also appear, particularly on the Gulf coast. Beaches where stingers are known post warning signs and, in a number of places, vinegar stations, which exist because vinegar is the recognised first response for certain sting types before medical help. Ask at the hotel, look for posted signs, and take a purple flag seriously. Any sting that involves severe pain, extensive marks or any breathing or cardiac symptom is an emergency and needs immediate medical attention, not a home remedy.

The more mundane hazards are more common. Sea urchins and sharp coral at the waterline on island beaches, which reef shoes solve entirely. Stonefish and other camouflaged fish in shallow rocky water, which is why nobody should walk barefoot on rock or reef. Sandflies on some beaches at dawn and dusk. And the sun, which at eight degrees north is stronger than almost any Gulf family expects, and which does most of its damage between eleven and three.

Boats and small craft

• Use licensed operators with passenger insurance in force. A boat found on a beach is not a licensed passenger vessel.
• Count life jackets before departure, in child sizes as well as adult, and have children wear them under way rather than stowed.
• Do not board an overloaded longtail. Overloading in a chop is the single commonest cause of small boat incidents on this coast.
• Confirm the skipper knows the route and has a radio or a working phone, and that somebody ashore knows the itinerary and the expected return time.
• Accept a cancellation. Harbour authorities suspend departures in poor conditions and a responsible operator will not sail against a warning, so never shop around for one who will.

Who to believe about the weather

The Thai Meteorological Department publishes the official forecasts and issues warnings for heavy rain, strong wind and rough seas, including small craft warnings that tell you when boats should stay in port. That is the authority to check, at tmd.go.th, rather than a general weather application which will show a rain icon for weeks on end and tell you nothing about the sea state. Local knowledge sits alongside it: the pier, the dive shops and the boat crews know their own water and will tell you honestly, because a bad day out costs them more than a cancellation does.

The rule we hold our own bookings to is simple. Any warning, from any of those sources, cancels the water. We move the day ashore, we do not put a family on a boat because the trip is paid for, and no client has ever complained about that decision afterwards.

It is worth saying why this needs stating at all. Families travelling in the Gulf summer have usually flown a long way, paid for a villa and a boat, and have a fixed number of days, and every pressure in that situation pushes towards going anyway. The pressure is compounded when a hotel desk or an informal operator, whose incentives run the other way, says the sea looks fine. Decide the rule before the trip, tell the children the rule, and let the office rather than the family be the one that cancels. A lost morning is a lost morning. The alternative is not comparable, and on this coast in the monsoon months it is not hypothetical either.

Check official forecasts, rough sea warnings and small craft warnings with the Thai Meteorological Department at tmd.go.th, confirm beach flag status and lifeguard hours with the local municipality and the beach lifeguard service on the day, verify vessel licensing and passenger insurance with the Marine Department of Thailand, and treat any instruction from lifeguards or harbour authorities as binding.

This briefing is general information, not legal, tax or investment advice. Thai rules change frequently and individual cases differ. Verify current requirements with the relevant authorities, including the Immigration Bureau, the Board of Investment, the Land Department, the Department of Business Development and the Revenue Department, and take advice on your own facts before acting.