Major water quality problems in Thailand's rivers are dissolved oxygen depletion, fish kills, high ammonia nitrogen, high coliform bacteria, and eutrophication phenomena. Approximately 43 million people in Thailand drink contaminated water, which can cause diseases like diarrhoea, typhoid, and dysentery.

The cleanliness of water in Thailand is a "split" reality: while municipal tap water in cities like Bangkok is treated to meet WHO standards at the plant, you should not drink it. This is because the aging pipe infrastructure can introduce contaminants like lead or bacteria before the water reaches your faucet. For bathing and brushing teeth, the tap water is generally fine for most travelers. For drinking, Thailand has a massive and highly efficient bottled water industry; "Singha" or "Nestlé Pure Life" bottles are available for just a few baht at every 7-Eleven. In 2026, many eco-conscious travelers also use filtered water refill stations located in hotels and malls. When it comes to beaches and islands, water quality varies; popular spots like Phuket or Koh Samui undergo regular testing, but after heavy monsoon rains, runoff can temporarily lower the quality. Always look for the "Blue Flag" certification or check local advisories if you plan to swim in lagoons or near busy ports where boat traffic might affect water clarity.