As in most developed cities, tap water is clean and drinkable. It is not the same in Thailand, we have to buy bottled water or use water filter at home.
Another difference is sparkling water is not common in Thailand. You will find only still water in convenient stores. Sparkling water in Thailand is available at fancy international restaurants or luxurious supermarkets.
We had a shocking experience about water when we went to Prague castle. Without knowing, we bought a small bottle of still water each and it was 100 Czech crown (140 Baht) per bottle. And the second time in a bar in a hotel where a bottle of still water was more expensive than a cocktail.