Carbonated water is not bad for you as long as there are no additives.
More Info: As with any food or drink, survey data can be found to corroborate both negative and positive side effects of consuming man-made carbonated water. For example, a survey conducted in 2005 revealed that respondents who drank carbonated water or soft drinks during the day were more likely to experience heartburn at night. However, because of the additional presence of caffeine and chemical additives, the correlation was much higher with soda pop than it was with carbonated water.
On the other hand, a big health benefit of regular carbonated water is the fact that it has no added sugar, colors, preservatives, or sweeteners. As a result, choosing to drink this kind of "soft drink" versus a cola or other soda pop equates to a diminished intake of potentially harmful ingredients.
An early method of making carbonated water was pioneered in the 18th century by Swedish scientist Torben Bergman. Luckily, his process of using chalk as a main ingredient has since gone by the wayside. Still, with the explosion of flavored carbonated waters, anything beyond club soda or soda water on the supermarket shelves these days tends more towards the soda pop ingredients side.
Yet another important distinction to draw is the difference between man-made and naturally occurring carbonated waters. The latter can be exceedingly expensive, and are often relied upon more as a hot springs healing source than for refreshment or hydration. However, these types of carbonated waters can sometimes have the added benefit of positive mineral ingredients. At the same time, if a man-made seltzer is not plain, it may have sodium, salt, and other ingredients that taint its purity as compared to regular H2O.