In general, drinking fluids should hydrate you, but there are actually many beverages that dehydrate you! Think of tea and how it makes you have to go to the bathroom to *ahem* relieve yourself right away - that's dehydrating! Also, many manufactured beverages contain a a ton of weird things that can cause dehydration when consumed in excess. Dehydration is so bad for your skin, so if you want that glowing, soft, hydrated skin, don't drink too much of this stuff:
The National Institute on Alcohol Abuse and Alcoholism confirms that alcohol is a diuretic (causes dehydration). Drinking too much of this stuff not only creates an electrolyte imbalance, but it can also prevent the body from being able to absorb water!
Though it has less caffeine than coffee, soft drinks are a source of the chemical. Caffeine is a diuretic, which means that it makes you pee!! Without replacing the fluids lost, excessive drinking of soft drinks can contribute to dehydration.
Like I mentioned above, caffeine can cause some serious dehydration if you consume too much of it. Try not to go over two and three cups of coffee each day to avoid your skin (and the rest of your body) to react to the lack of fluids in your system.