I'm pretty sure you're looking for the word "preventative". You treat the water in the hopes that the treatment will prevent the water from making you ill, but it's still possible that drinking the water would not make you ill even without the treatments.
Your question about Cold-fX, though, doesn't carry the same meaning. You take Cold-fX after you get a cold. It's impossible to tell if the Cold-fX actually helps with your recovery. We wouldn't explain that by talking about it as a property of the medicine itself. We'd describe it by talking about our efforts to determine whether Cold-fX helps with our recovery, and those efforts are inconclusive (and the claim that Cold-fX does help is, at best, dubious, meaning there's reason not to trust it).
If a person does believe that Cold-fX is helping with her cold but in reality it isn't at all, that would make Cold-fX a placebo. You say placebo isn't exactly the word you want, but I think it is. If you decide to take Cold-fX for your cold, for example, you'd naturally consider it medicine. You're probably taking it because you're convinced it will help, right? Someone who knows it's not really helping you would call it a placebo, and that's probably the very same word you yourself would use if you learned after the fact that Cold-fX didn't help your cold at all.