There is little question that water is water, whether that water comes from the tap, from soft drinks, or from fruits and vegetables. The differences are all a matter of (1) how readily you are willing to get your hydration one way versus another, and (2) what comes wrapped up with the water in your chosen method of hydration.
With regard to (1), clearly, if your predilections are such that you're going to only drink 2 cups of tap water per day versus 8 cups of Crystal Light, then hydration needs are best served by the latter.
With regard to (2), there are a whole set of considerations which depend specifically on where you're getting your hydration from. For example, Diet Coke contains other things in addition to water, such as caffeine. Caffeine is a diuretic and therefore works against hydration. Crystal Light (powder) contains Aspartame, which is hazardous if consumed in outrageously large quantities, so that is a consideration. Our tap water, here in town, sometimes contains traces of dangerous chemicals, so even tap water has its detractions.
For me, (1) is an overriding consideration. The additives and accoutrements in my chosen means of hydration are manageable risks, but the fact that I don't drink enough to stay sufficiently hydrated, if I limit myself to tap water, is an incontrovertible risk. YMMV.
