Urban myth and science disagree on the answers to this one!
Science says: A drink like crystal light is so very low in calories, that it essentially counts as water. Yes...there are a few calories in it, so if you drink say 250 mL (1 cup) of water, then you can only count about 230 mL for your water, which is pretty much all of it. (I know a few members here will disagree with this vigorously, but I am basing this on journal articles from scientific journals that I have come across). This actually includes things like herbal tea (as long as no sugar and cream are added), diet soft drinks, etc.
Part of it is a mental thing. If we become accustomed to sweet drinks, then in the absence of artificially sweetened ones, we will probably reach for regular pop or Kool-Aid instead of water. Drinking plain old water is just a good habit.
Caffeinated drinks is an exception, but don't believe the urban myth that caffeine makes your body excrete more water than you take in with the drink. For example, in an average person for an average coffee, about 1/3 -1/2 of the water will be taken in as water, while the other 1/2 - 2/3 will be excreted due to caffeine action in the kidneys. (I pity the research assistants that had to help with this study!
I personally count my low-cal or 0 cal drinks as water. I have successfully been losing weight and never feel dehydrated. So, I know I'm not exactly doing what WW says, but hey...whatever works. I hate the taste of water in this area, and crystal lite costs less than bottled water here.