Mayo Clinic:
http://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/expert-answers/caffeinated-drinks/faq-20057965
I’ve been seeing ads that say caffeinated drinks hydrate you as well as water does. Is this true?
It is true that caffeinated fluids can contribute to your daily fluid requirement.
Drinking caffeine–containing beverages as part of a normal lifestyle doesn’t cause fluid loss in excess of the volume ingested. While caffeinated drinks may have a mild diuretic effect — meaning that they may cause the need to urinate — they don’t appear to increase the risk of dehydration.
Still, caffeinated drinks can cause headaches and insomnia in some people. Water is probably your best bet to stay hydrated. It’s calorie–free, caffeine–free, inexpensive and readily available.
WebMd
http://www.webmd.com/balance/caffeine-myths-and-facts?page=3#2
Caffeine Myth No. 5: Caffeine Has a Dehydrating Effect
Caffeine can make you need to urinate. However, the fluid you consume in caffeinated beverages tends to offset the effects of fluid loss when you urinate. The bottom line is that although caffeine does act as a mild diuretic, studies show drinking caffeinated drinks in moderation doesn’t actually cause dehydration.
Academic Research
http://journals.lww.com/acsm-essr/Abstract/2007/07000/Caffeine,_Fluid_Electrolyte_Balance,_Temperature.8.aspx
Caffeine, Fluid-Electrolyte Balance, Temperature Regulation, and Exercise-Heat Tolerance
Dietitians, exercise physiologists, athletic trainers, and other sports medicine personnel commonly recommend that exercising adults and athletes refrain from caffeine use because it is a diuretic, and it may exacerbate dehydration and hyperthermia. This review, contrary to popular beliefs, proposes that caffeine consumption does not result in the following: (a) water-electrolyte imbalances or hyperthermia and (b) reduced exercise-heat tolerance.
Snopes
http://www.snopes.com/medical/myths/8glasses.asp
Regular coffee and tea drinkers become accustomed to caffeine and lose little, if any, fluid. In a study published in the October issue of the Journal of the American College of Nutrition, researchers at the Center for Human Nutrition in Omaha measured how different combinations of water, coffee and caffeinated sodas affected the hydration status of 18 healthy adults who drink caffeinated beverages routinely. “We found no significant differences at all,” says nutritionist Ann Grandjean, the study’s lead author. “The purpose of the study was to find out if caffeine is dehydrating in healthy people who are drinking normal amounts of it. It is not.” The same goes for tea, juice, milk and caffeinated sodas: One glass provides about the same amount of hydrating fluid as a glass of water. The only common drinks that produce a net loss of fluids are those containing alcohol — and usually it takes more than one of those to cause noticeable dehydration, doctors say.