Q. Why does coffee, tea and alcohol dehydrate me?
A. The reason for this is that caffeine and tannins in coffee and tea respectively, are connected to the minerals that boost water excretion via the kidneys. These minerals include sodium, potassium, magnesium and calcium. Therefore making the cells dehydrated. Alcohol is processed by a similar mechanism. All these drinks not only use up water during metabolism, but also act as diuretics, increasing the amount of water excreted so that you end up with a deficit .
References:  Armstrong LE, Pumerantz AC, Roti MW, Judelson DA, Watson G, Dias JC, Sokmen B, Casa DJ, Maresh CM, Lieberman H, Kellogg M. Fluid, electrolyte, and renal indices of hydration during 11 days of controlled caffeine consumption. PubMed PMID: 16131696.