Recently I read an article in a medical journal that denounced the old theory that 'coffee doesn't count as water, because the caffeine contained in coffee acts as a diuretic". They have now apparently shown that the diuretic effect (which makes your kidneys eliminate more water) is not enough to counter the benefits of taking in the fluids while you're drinking the coffee.
So, yes, I believe that anything you drink counts as water--even beverages which contain calories, such as milk or juice-- because they put fluid in your system. Whether it's 8 cups of water versus 8 cups of soda definitely has an effect on your system in other ways, but as long as you drink plenty of 'fluids' you should remain well hydrated!