You are talking about two kinds of parasites; internal and external. The only medication I know of that may help with both is Ivermectrin and it's not recommended for continual use. It's available at feed stores but I wouldn't use it as a preventative, only a cure.
I've not heard of putting stuff in their water but you can feed chickens pumpkin and other winter squash which seems to help as a preventative for worms. Others use food grade (ask for it at a feed store or order online) diatomaceous earth, DE, in their food on occasion to help with worms and other internal parasites. Do not use the kind available at pool supplies; it must be food grade.
DE can also be used as a preventative for external parasites as well. I use it in the coop by dusting their new bedding with it and I make a whitewash of DE and water and paint their roosts. You can dust around their pens and runs to keep the flies down as well.
Other preventatives are simply good husbandry. Keep the chicken areas clean. Pick up poops from nighttime roosting and rake out their run on occasion.
The only thing I add to their water is organic, Braggs apple cider vinegar (ACV) to help keep their systems in a good balance. One capful per gallon of water in a plastic waterer works well.
Mary