From the Gulf to the Andaman: what is different about this water.

Why the two seas behave differently

The Arabian Gulf is a shallow, almost enclosed basin, connected to the Indian Ocean through a single narrow strait. It is mostly tens of metres deep, it is largely sheltered from ocean swell, and it heats up like a bath through the summer. The Andaman Sea is an arm of the Indian Ocean: deep offshore, open to weather systems generated thousands of kilometres away, and subject to a monsoon that reverses twice a year. That difference in basic geography produces every practical difference a family will notice, and none of them is a reason for anxiety, only for briefing.

The Gulf of Thailand, on the eastern side of the country, sits between the two: shallower, more enclosed, gentler, and in feel much closer to the water Gulf families already know. Families who want the smallest adjustment for young children or nervous elderly swimmers should be steered to Samui, Hua Hin or Cha-am rather than to the Andaman coast, and that is a large part of why we send summer families east.

Temperature

Andaman sea temperatures run roughly twenty eight to thirty degrees through most of the year, which is genuinely warm water by any global standard. It is nonetheless cooler than the Arabian Gulf in high summer, where inshore water can climb well into the thirties and feels close to body temperature. The first reaction of many Gulf visitors is that Thai sea water is refreshing; the second, an hour later, is that children get cold in it in a way they never do at home. Small children lose heat quickly even at twenty nine degrees, particularly in wind or after rain, so watch for shivering and blue lips, bring towels and dry clothing to the beach rather than leaving them at the villa, and limit sessions to shorter periods than a family would expect.

Tides, which are the biggest surprise

Tidal range in the Arabian Gulf is modest in most places. On the Andaman coast it is substantial, and in Phang Nga Bay and around Krabi it can run to two or three metres between high and low water, changing the beach completely twice a day. Practically, that means the sea can retreat hundreds of metres at low water, exposing sand flats, rock and coral where you swam that morning. Sandbars appear and disappear. A boat that came alongside a pier at noon may need a wade at four. Beaches that are safe and swimmable at high tide can be too shallow, or too rocky, to enter at low.

Tide tables are published for each area, and every hotel front desk, dive shop and boat operator has them. Reading them takes a minute and it structures the day: swim around high water, plan boat departures and landings around the state of the tide, and do not let children walk out to a sandbar without knowing whether the water is rising. The classic and genuinely dangerous mistake is walking out at low tide to an exposed bar or a small island and finding the channel behind you filled and running on the way back. Ask locally before any such walk, and if in doubt, do not do it.

Swell, shore break and current

The Arabian Gulf has short choppy wind waves. The Andaman has ocean swell, which is a longer, heavier, more powerful thing, generated far away and arriving whether or not there is wind on the beach in front of you. In the monsoon months, from May to October, that swell reaches the west facing beaches of Phuket and Krabi and produces surf, a strong shore break where waves break directly on a steep beach, and the rip currents that our sea safety guide treats in full. A shore break can knock an adult over and roll a child, and on steeper beaches such as Surin it can be significant even on a day that looks calm from the road.

Current is the other new variable. Water moves around headlands, through gaps between islands and in and out of bays with the tide, and it can be strong enough that a competent swimmer makes no progress against it. This matters most when snorkelling from a boat, which is where visitors most often get into trouble because they are absorbed in what they are looking at and drift steadily downwind without noticing. Swim upcurrent first and drift back, keep the boat in sight, and use a float or a life jacket for anyone who is not a strong swimmer. On the Andaman coast, snorkelling in a life jacket is entirely normal and nobody will think anything of it.

What lives in it

The Andaman is far richer than the Arabian Gulf, which is part of its appeal and the source of a few new rules. Living coral is sharp and cuts easily, and cuts from coral are slow to heal and prone to infection, so they should be cleaned properly and watched. Sea urchins sit in rock crevices in shallow water. Stonefish and scorpionfish are camouflaged and rest on the bottom, which is the reason for the standing rule that nobody walks barefoot on rock or reef. Jellyfish appear seasonally and are covered in our sea safety guide. Reef shoes for everybody, adults included, remove most of this at a stroke, and a simple habit of looking before putting a foot down removes most of the rest.

None of this makes the water dangerous in ordinary terms. Thailand's beaches are swum by millions of people a year without incident. It simply means that the sea here contains things, whereas the sandy shallows of a Gulf corniche largely do not, and children should be told that before they are told to enjoy themselves.

What children should know before the first swim

• Swim between the red and yellow flags where lifeguards are on duty, and never in the water when a red flag is flying.
• Never swim alone and never behind a headland or out of sight of an adult on the beach.
• If you are pulled out to sea, do not fight it. Float, stay calm, and swim sideways along the beach until you are free, then come in.
• Wear reef shoes on rock and coral, and never put a hand or a foot somewhere you cannot see.
• Do not touch or stand on coral, and do not chase or feed the fish.
• Come out when you are cold, and drink water often even though you are in it.
• Tell an adult before going back into the water, every single time.

Sun, skin and the rules of the parks

Phuket sits around eight degrees north of the equator, and the sun there is stronger than a Gulf summer suggests, because the heat at home comes with a haze and a very high sun angle at noon that people instinctively avoid, while a breezy beach day here disguises the burn until the evening. Shade, hats and long sleeves outperform any cream, and the middle of the day belongs indoors or under cover. This is doubly true for anyone who has been in an air conditioned Bangkok hospital for a fortnight before travelling to the coast.

One rule catches families out. Thailand has prohibited sunscreens containing certain chemicals, including oxybenzone and octinoxate among others, in its marine national parks, because of coral damage, with penalties for breach. Since many of the islands worth visiting are inside those parks, it is worth checking the ingredients before packing and choosing mineral based products, which are widely available in Thailand. Confirm the current list with the parks department if you are uncertain, and take the reef rules seriously in general, because the boat crews do.

Confirm tide tables and local sea conditions with your hotel, the pier or the boat operator on the day, check official warnings with the Thai Meteorological Department at tmd.go.th, treat lifeguard and flag instructions as binding, and check the current sunscreen restrictions, entry fees and reef rules for marine parks with the Department of National Parks, Wildlife and Plant Conservation before travelling.

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.