How often have you heard that coffee is dehydrating? THIS IS FALSE.
The truth is your cup of joe actually does the opposite. Many used to believe that they were dehydrating, but that myth has been debunked. The diuretic effect does not offset hydration. Researchers at the University of Birmingham in the U.K. studied the fluid levels of 50 men who had a habit of consuming about three to six cups of coffee each day. With this kind of moderate coffee consumption, the authors conclude that "coffee ... provides similar hydrating qualities to water."
So go head and keep the coffee coming!