Soda Water is one of those funny liquids. Is it water? Is it a soda? Somehow it must be a combination of those two things, right?
Soda Water is known by many names - club soda, sparkling water, seltzer water. Really, soda water is regular water that has had carbon dioxide pumped into it. This is the same thing that they do to soda liquids to make them bubbly.
While these water drinks do not have alcohol in them, they are very commonly used as a mixing agent for cocktails. They give the alcoholic drink a little spritz and mouth appeal.
You can even make your own soda by mixing soda water with a flavoring. That in essence is all that soda is.
Soda water in all of its forms is completely low carb. There are no carbs at all in water. There are no carbs in carbon dioxide :) Carbon dioxide is just as gas, and is fact the gas you breathe out of your body. You breathe in oxygen, you breathe out carbon dioxide.
However, make sure that you do not confuse soda water with TONIC water. Tonic water is water flavored with quinine, sugar and fruit extracts. Tonic water most definitely does have carbs and calories.
Lisa Shea's Library of Low Carb Books