It's just like all the other "diet" water drinks out there: purified water mixed with electrolytes, some vitamins or phytochemicals, flavors, colors and presumably artificial sweeteners. Different flavors offer different "benefits", such as "antioxidants", "natural energy", and, get this, "appetite control". My favorite bit is a quote from Emily Mitchell, MS RD CSSD CDE. "After reviewing the ingredients, I believe Skinny Water is an excellent, sugar-free and healthy alternative to traditional, high-calorie carbonated soft drinks."
I appreciate her qualification here, that this brand of colored water is a better alternative than soda. Likely she would agree with me, that ordinary clean water is the best option of all. Why pay someone for the privilege of drinking a bunch of chemicals when you can drink purified water for nearly free? (Yes, figuring in the cost of tap water and a Brita filter.) Why think that a freakishly pink drink is going to hydrate you better than water?
Ah, but you say, "I just don't like the taste of normal water." Then you have a couple of great, all-natural options. Do like my clever former roommate and add a splash of juice to your water. Cran-grape, apple, orange--they'll all add just a touch of flavor and some natural electrolytes while providing just a few calories. If a post-workout recovery drink is what you need, there's a great recipe over at No Meat Athlete, based on dates (great source of potassium), citrus fruit, and sea salt. Or tea. Black, green, or herbal: all have been drunk for centuries, and provide all kinds of interesting phytochemicals. Check for organic varieties.