Carbonated water is merely water with carbonation added and is just as hydrating as water without carbonation.
More Info: Many carbonated beverages, such as sodas, beer, and tonic water typically have added sodium, sugar, and calories. As an example one energy drink contains nearly two tablespoons of sugar and three times the amount of caffeine than a can of cola. Seltzer water, or purely carbonated water, usually has no other added ingredients. The carbonation process does not alter the water in such a way that hydration does not occur. Therefore, drinking carbonated water can hydrate you.
Fact: Any beverage with caffeine acts as a diuretic and causes the body to become depleted of water stores. Carbonated water with added caffeine will therefore produce a dehydrating effect.
Fact: It is important to sip water throughout the day, as the human body needs a steady supply of water at all times. Don't overdo it. Drinking too much water, though difficult to do, is dangerous and even lethal.
Fact: Signs of dehydration include lightheadedness, dizziness, and fatigue. Dehydration can range from mild to severe and is best avoided by staying continually hydrated. Carrying a water bottle at all times can help to develop the habit of taking in plenty of water throughout the entire day.