Coffee is not dehydrating:

Coffee Myth-Busting: Cup Of Joe May Help Hydration And Memory : The Salt : NPR

I lived on coffee, beer and potato chips in my mid-20's; one of the things I would definitely change if I could go back and do it again. I would get a genetic test to see how you metabolize caffeine though; I'm a "slow" metabolizer and drinking too many cups could/may have (ah, youth!) hurt my heart.

http://www.geneplanet.com/genetic-an...etabolism.html