I don't consume caffeine, but Dave Arnold of the (unimpeachable) Cooking Issues podcast has a whole spiel about how Diet Coke drinkers use it as a primary source of hydration.
Im kinda that way with coffee. Eventually I switch to water but sometimes its the only thing I drink in the day.
In fact I think many people primarily drink something other than water. And im not saying thats good. But between pop, coffee, tea, juice, alcohol and an aversion to tap water, you have a lot of people who rarely drink water.
