Some folks, even doctors, say that only water "counts," but when asked for scientific evidence that proves the distinction, those folks are not able to come up with any. Most especially, water from water-laden foods is especially unassailable: Such water unquestionably supports hydration, on an ounce-by-ounce basis with drinking water.
What we do know is that there are some things in some zero-calorie beverages that have effects that you may or may not want. By the same token, if the choice is between incurring those risks versus not drinking enough, the best approach would very likely to be to incur those risks. In other words, if you need Crystal Light or Diet Caffeine-Free Coke to support your hydration, and the alternative for you would be to be less hydrated, then surely use those tools.
Note that I specified that these alternative water sources must be decaffeinated. Again, going back to the scientific evidence, caffeine is a diuretic, and therefore does not support hydration on an ounce-by-ounce basis with drinking water. Rather, caffeinated beverages "count" significantly less than ounce-by-ounce (but more than half -- the precise amount depends on which research you wish to rely on).
In a perfect world, no one who is overweight would ever eat sugar, no one (at all) would ever eat a stitch of hydrogenated fats, etc., but we live in the real world and we have to do what's best given our own circumstances.