In a long list of countries where it is not safe to drink water, figure all South American countries and Mercosur members with the exception of Chile.
The list elaborated on data from the United States Centers for Disease Control and Prevention, with suggestions for each country also includes in the bad list, the Falkland Islands, South Georgia, but exempts St Helena.
In effect, most travel magazines from the United States and European Union publish recommendations on drinking water for travelers, that is where you can and where you can't drink the tap water. And points out that rather than hearsay reports from other travelers or results of a hasty web search, the publications indicate the safest way to learn about the water quality in countries, travelers planning to visit a certain country should refer to the reports from CDC.
This can be checked visiting the CDC at Destinations webpage, Stay Healthy and Safe with a reference in depth advising on the different countries.
The much shorter CDC List of countries where it is safe to drink the water, can be easily checked, however at the other end, those countries where it is not safe to drink water or even wash your teeth includes surprisingly, countries from South America with a strong tourism industry, and at times a healthy reputation.
This includes Mercosur partners, Uruguay, Argentina, Brazil, Paraguay, plus Colombia, Ecuador, Peru, Bolivia, Venezuela and French Guiana, Suriname and the Cooperative Republic of Guyana.
Chile according to CDC is where it is safe to drink the water.
And as mentioned above, Falkland Islands, South Georgia and South Sandwich Islands figure in the 'don't drink the Water”, while apparently there is no problem in St Helena.