Despite the fact that the authorities have made efforts to make tap water meet World Health Organization standards, very few people drink tap water in Thailand, even the local population. Bottled water is widely used instead. 

Some people actually boil tap water before use, but this will not remove chemical toxins or remnants of whatever else was there before boiling. You should also be careful with ice, as freezing does not protect you from bacteria, viruses or chemicals. Brushing your teeth with tap water is considered to be safe, although those with very sensitive stomachs may occasionally experience problems.

In restaurants, you will find the water to be generally safe. You can always buy small bottles if you like but make sure the seal has not been broken.

However, you should be very careful with street vendors and street food stalls. The biggest risk is actually from the cleanliness of the glasses themselves. You can become very ill indeed if you are not careful. Drink directly from the bottle if you are in any doubt.

Don't worry too much about the ice that is served in cafes etc as they usually have the ice delivered to them from government inspected ice factories.