Functional waters are any brand of H2O “enhanced” with special ingredients, like herbs or antioxidants, that supposedly bring health benefits. Some types make sense to drink: Water with added electrolytes may be useful if you exercise a lot and sweat heavily and need to replenish sodium and other electrolytes quickly, or if you have a diarrheal illness and are potentially losing electrolytes that way. And waters with added vitamins can give you a nutritional boost (though you absorb vitamins better when you get them through food).
Be more skeptical of any brands that claim to alter the body’s pH, as well as ones that have added hydrogen to purportedly load you up with extra antioxidant power. There’s no credible science to back these claims. One 2012 lab study found that alkaline water with a pH of 8.8 neutralized pepsin, a stomach enzyme involved in breaking down food proteins and producing stomach acid, which suggests it might help soothe acid reflux—but it hasn’t been studied in people yet. For the most part, your body is designed to maintain its own pH balance naturally, and what you eat or drink doesn’t change that.