If you look at the ingredients of VitaminWater 10 (owned by Coca Cola), you might be pleased to see that it contains the natural sweetener stevia. However, you will also notice that it is loaded with crystalline fructose, sucrose, and a mysterious product called Erythritol.
Erythritol is a sugar alcohol, a sweetener that does not provide as many calories as sugar. But the reason that sugar alcohols provide fewer calories than sugar is because they are not completely absorbed into your body. For this reason, high intakes of foods containing sugar alcohols can lead to abdominal gas and diarrhea.
Also, bear in mind that while sugar alcohols are lower in calories, gram for gram, than sugar, they are not calorie-free, and if eaten in large enough quantities, the calories can be comparable to sugar-containing foods.
Some sugar alcohols are reasonable to consume in moderate quantities -- for example, xylitol doesn't spike blood sugar levels in the way that high-fructose corn syrup might, and it is anti-bacterial and actually helps prevent dental cavities.