The Ministry of Natural Resources and Environment has just completed their 2021 Environmental Quality Audit of Thailand, and Prachuap Khiri Khan and Koh Samui dominated the list of cleanest sea water beaches.
The Weddell Sea has been claimed by scientists to have the clearest waters of any ocean in the world. Described by a historian as “the most wretched and dismal region on earth”, due to the flash freezes that caught Shackleton's ship, its clarity is only belied by the sheer depth of the ocean below.
Bottled water is very cheap to buy at the numerous 7/11 stores etc. Stick to bottled water for drinking. It is available everywhere and not expensive. However, the water is safe enough for you to have a bath, wash your hair, brush your teeth.