I think that potable water is safe to drink from most countries. My airline does not pick up potable water from certain countries and as an airline we don't serve it in-flight as drinking water.
When I first started flying we used to use water from the potable supply but that has not happened for many years now; but I think this is more to do with customers and the general public actually wanting bottled water rather than because the potable water is not safe. It is more of a fashion thing, as also seen in restaurants nowadays, as years ago we would have had a jug of water in a restaurant also.
Potable water is specifically treated to make it safe but that can actually cause it not to taste too good and that is why airlines now mostly use bottled water. Potable water is however still used for the hot beverages that are made using the bev makers in the galleys.