Colloidal Silver kills germs but it's not going to purify water necessarily. If there's metal/gas poisoning, radiation, pharma products, etc - in other words any kind of toxic material that's not germ based, CS won't help. To put it crudely, would you drink from a jar with a turd in it even if it were full of Colloidal Silver ?
If you have no choice other than questionable water sources, food grade Iodine is the best way of sterilizing in a pinch. That's why so many missionaries, people on safaris etc carry Iodine with them.
But a trusted water source is obviously the way to go.