Not all water is created equal
The ability for the mouth to neutralize acids is attributed to the salivary glands.  The salivary glands secrete fluids to combat the acidity caused from drinking and eating foods and liquids that are acidic.  These commonly ingested foods and liquids cause the oral environment to have a sustained low pH that can erode and demineralize teeth, sometimes causing irreversible damage.  The acidity of the diet in addition to providing a carbohydrate rich food source to bacteria can cause a long sustained low pH.  Constant eating or drinking acidic foods and liquids throughout the day is putting the teeth at risk for tooth decay. Bacteria thrive in an acidic environment as they consume carbohydrates they replicate and give off acid as a byproduct of their carbohydrate consumption. Leaving acidic biofilm debris on teeth causes micro-cracks in the enamel allowing further ingress of bacteria and acidic biofilm inside the tooth which can often not be removed with simple brushing  alone.  
One would assume that when drinking one of the many different bottled waters on the market that they are getting very clean drinking water without any additives. However, many of the different bottled waters are not offering neutral pH water. The photo above illustrates an extensive variation in bottled water available today.  Understand that drinking bottled water all day long should ideally not cause harm to the body or create a higher risk potential for tooth decay, but it can.  Drinking various types of beverages like, fruit juice, soft drinks, energy drinks, tea and coffee or even acidic foods can have very harmful repercussions on teeth because they have lower pHs and more carbohydrates to feed harmful bacteria that cause sustained lower pH levels in the mouth.