Most of Thailand's monitored coastal water is rated "fair" or "good": in 2025 the Pollution Control Department (PCD) put 91 % of its 210 sites in those two classes, with the Andaman coast cleanest and the inner Gulf near Bangkok the worst. The real day-to-day risk at resort beaches comes from drains and river mouths after heavy rain, which you can avoid.
We found no official beach-by-beach bathing grade for Thai resorts. What exists is a national monitoring program, a yearly report and occasional inspections when something goes wrong, such as an algae bloom. This page explains what those official results say, what they do not say, and the simple habits that keep you out of the dirtiest water. It is general information; for symptoms after swimming, see a doctor or pharmacist.
Thai coastal water: the official picture
- Who monitors
- Pollution Control Department (PCD), Ministry of Natural Resources and Environment
- Monitoring points
- 210 coastal sites nationwide
- Scale used
- Marine Water Quality Index (MWQI): good, fair, deteriorated, very deteriorated
- 2025 result
- Good 31 %, fair 60 %, deteriorated 6 %, very deteriorated 3 %
- 2024 result
- Good 49 %, fair 43 %, deteriorated 6 %, very deteriorated 2 %
- Main problems
- Nutrients and bacteria in some areas, linked to algal blooms
How the PCD tests Thailand's coastal water
The PCD samples water at 210 points along both coasts and combines the results into the Marine Water Quality Index. Each site is then classed as good, fair, deteriorated or very deteriorated. The PCD's own website could not be reached when we checked in October 2026, so the figures on this page come from Thai news reports quoting the department's director-general, with the date of each report.
The index is a summary for a whole site over a year, not a daily swimming forecast. A beach rated "fair" can still have a bad day after a storm, and a "deteriorated" site may be a river mouth or a port rather than a swimming beach. Read the results as a guide to which coasts are generally cleaner, not as a guarantee for the day you swim.
The 2025 results: best and worst sites
Reporting the 2025 results in December 2025, the PCD said the ten-year trend from 2016 to 2025 was fairly stable and that the Andaman coast had consistently been the cleanest. The ten best sites in 2025 were, in order (Bangkok Biz News, 31 December 2025):
- Ao Sapli, Chumphon
- Ao Bo Muang, Krabi
- Hat Khlong Dao, Koh Lanta, Krabi
- Hat Thung Wua Laen, Chumphon
- Ban Khlong Nin, Koh Lanta, Krabi
- Hat Rin, Koh Phangan, Surat Thani
- Hat Thai Mueang, Phang Nga
- Hat Lamai, Koh Samui, Surat Thani
- Koh Kai, Krabi
- Ao Bang Son, Chumphon
The five worst were all at river and canal mouths around the upper Gulf: the Chao Phraya mouth and two other sites in Samut Prakan, Bang Khun Thian in Bangkok, and the Tha Chin river mouth in Samut Sakhon. These are not tourist beaches. Two of the best sites are on Koh Lanta, and Koh Kai is a regular stop on boat trips from Krabi; our Ao Nang guide describes how the water there changes with the weather.
The 2024 results looked better on paper, with 49 % of sites rated good, against 31 % a year later. That year's best list included Samila Beach in Songkhla, Nai Harn in Phuket, and Ton Sai and Maya Bay in Krabi. The worst five were again in the inner Gulf and included the area south of Laem Chabang, the industrial port in Chonburi (Naewna, 7 January 2025, quoting the PCD).
Pattaya and Phuket: what is known
We found no official PCD figure for individual Pattaya beaches, and we do not rank them without one. Pattaya Beach and Jomtien appeared on neither the best nor the worst list in 2025. The 2024 worst list named south Laem Chabang, which is a container port up the coast, not a Pattaya swimming beach. For the beach south of the city, our Jomtien Beach guide covers a drainage incident in early 2026 and what officials said afterwards; the Pattaya guide covers the rest of the resort.
Patong, Phuket's busiest beach, scored 65 on the index in 2025, which is "fair", with nitrate and phosphate as the problem indicators. On 6 February 2026 green and red algae appeared in the water and on the southern end of the beach. The PCD's regional office inspected it and found the water within the coastal standard for recreation, and the municipality cleared the algae every morning (The Bangkok Insight, 7 February 2026). Nai Harn, at the island's southern tip, was on the 2024 best list. Our Phuket guide covers the other beaches.
Green or brown water is not always sewage. Algal blooms, fed by nutrients from land, can color the sea and leave weed on the sand. At Patong in February 2026 the water still met the recreation standard, but if the sea looks or smells wrong, swim somewhere else that day.
After heavy rain: drains, rivers and runoff
Rain washes whatever is on the streets and in the canals into the sea. That is why the cleanest-looking resort beach can have a brown plume off a drain or a river mouth after a storm, especially in the rainy season and in built-up bays.
We found no official Thai rule on how long to wait after rain before swimming, and the WHO figure sometimes quoted online could not be checked against a WHO page. The sensible habits are simple:
- Swim well away from drain pipes, concrete channels and dark streams running across the sand into the sea.
- After a heavy downpour, give the water time to clear, and choose a beach away from river mouths.
- Do not swallow seawater; the US CDC gives the same advice for Thailand, along with swimming only in designated areas and following lifeguards' flags.
- With small children, pick the stretch of beach furthest from any outflow.
Rain brings a second, separate risk to the shallows: the DDC says jellyfish are more common after rainfall. Our page on box jellyfish in Thailand covers that, and the rip currents guide explains the beach flags.
Cuts, ears and stomachs: possible effects of dirty water
Polluted water mainly causes minor problems, but open wounds need care: coastal water carries bacteria that can infect cuts, which our guide to skin rashes and infections explains in detail. If you get an earache, a spreading rash, or diarrhea after swimming, see a pharmacist or a doctor and mention where you swam; the page on diarrhea in Thailand covers when a stomach upset needs a doctor.
Where to find clearer water
The pattern in the official results is consistent: the Andaman coast and the islands are cleaner than the mainland coast near Bangkok. If clear water matters to you, plan your beach days on the Andaman side between November and April, or on islands rather than the mainland. Our guide to the best time for the Thai islands explains the two coasts' seasons.
Frequently asked questions
Is the sea in Thailand clean enough to swim in?
Generally yes. In 2025 the PCD rated 91 % of its 210 coastal sites good or fair, with the Andaman coast cleanest. The worst sites are river mouths around the upper Gulf, not resort beaches. Avoid drains and river mouths, especially after heavy rain.
Is the water at Pattaya polluted?
No official PCD figure for individual Pattaya beaches was available when we checked, and Pattaya was on neither the best nor the worst list for 2025. The Gulf coast near Bangkok is generally less clear than the Andaman coast; stay away from drain outlets, particularly after storms.
Which Thai beaches have the cleanest water?
In the PCD's 2025 results, the top ten sites included Ao Sapli and Thung Wua Laen in Chumphon, Khlong Dao and Khlong Nin on Koh Lanta, Hat Rin on Koh Phangan, Lamai on Koh Samui and Koh Kai off Krabi. The results change from year to year.
Should I avoid swimming after rain in Thailand?
Be careful. Rain carries runoff from streets and canals into the sea, and the DDC says jellyfish are more common after rainfall. We found no official waiting time, so stay away from drains and river mouths and give the water time to clear.
Sources
- Thansettakij – PCD coastal water quality results 2025 (26 December 2025, Thai) (checked October 2026)
- Bangkok Biz News – The ten best coastal water sites in 2025 (31 December 2025, Thai) (checked October 2026)
- Naewna – PCD coastal water quality results 2024 (7 January 2025, Thai) (checked October 2026)
- The Bangkok Insight – PCD inspects algae at Patong Beach (7 February 2026, Thai) (checked October 2026)
- Thansettakij – Patong Bay water quality index 2025 (Thai) (checked October 2026)
- CDC – Travelers' health: Thailand (checked October 2026)