It's a worldwide correlation. But of course communism is not the only factor at play here. Population density, total population, size of country, and so on. They all play a part. In North America the correlation is skipped because these other factors are so vastly different between the countries. In South America not so much. Except for Brazil.
There's exactly one "communist" country on earth, which is North Korea. And that happens to be quite good on this metric.
Whatever correlation you are imagining in South America is completely spurious, even with your litany of reasons to exclude anything that doesn't fit your worldview. Most every government on the continent has switched between left- or right-leaning governments at least once in the last decade, and I assure you tap water quality changes much slower.
The Scandinavian countries that are frequently cited as examples any advanced nations' left strives to imitate are obviously killing it on drinking water quality, as well.
