The need is for the caffeine--the easiest way to get off the diet soda train is green tea, and to ease off the caffeine from there. Others just don't find water satisfying for other reasons, which I can relate to. I tend to drink a 60/40% mix of seltzer and fruit juice, which after a week or so adapting tastes surprisingly sweet. In any case, there are options beyond water that aren't all bad.