I believe that it used to be thought that the caffiene in coffee acted as a diuretic, which makes you go to the bathroom more often. Now, though, there has been research that only in large amounts does caffiene act as a mild diuretic. In fact, they suggest that coffee hydrates you instead of dehydrates unless you drink too much of it. Drinking alot of water is always a good thing for you no matter what else you drink, but if you are dehydrated you need to drink more water than usual. I actually heard that once you are thirsty, you are already dehydrated.