http://www.mic.hawaii.edu/dev_tech/biology/caffeine.html

Dear WW: above is one study that says caffeine beverages cause net water loss.
This may be true in the; beginning, but I think they failed to take into account
the decrease in water loss in people who drink coffee regularly.
I have known many hospital workers who drank many cups of coffee
during the working hours, and never developed dehydration. I doubt that they
also drank a lot of water.
Of course, caffeine containing beverages should not be a large fraction of
daily fluid intake.