Alcohol is a diuretic which means it makes you urinate more than normal. Alcohol is actually a kind of poison or toxin and after drinking massive amounts of alcohol, the body goes into a major effort to "wash it out" of the blood stream. This takes a lot of water. It is not exactly the chemical process of breaking down the alcohol that uses the water, but the mechanism of carrying the toxins across the membranes in the kidneys. This takes lots of water. So it is the body's removal of ethanol with water that leads to dehydration. Have you ever noticed you have to urinate a lot if you drink massive amounts of alcohol? That is, you get "pissed". This is your body removing the alcohol, and along with it, out goes a lot of water. Hence the smartest thing to do when you drink alcohol, is to have a glass of water with every drink.