A doctor told me that if you don't have a water filter, you are the water filter. That being said, the difference between the quality of most municipal tap water and bottled water is marketing dollars. In some countries, it is a fact that you have to boil the water before drinking it. I was so nervous about avoiding sickness on my last trip to a Latin American country that I used bottled water (on my doctor's recommendation) to rinse out my toothbrush and avoided salad.