The reality is drinking water in any place foreign to your system can cause a case of the trots. It doesn't always have to be "bad" water, the fact is water from different areas has different bacteria in it. Your system is used to the bacteria native to your local water, a different bacteria can cause issues without it necessarily being unhealthy water. Certainly there are many places in the world where water treatment isn't what it should be and can be infected with parasites, so it is best to avoid drinking tap water in any third world nation, and sticking to bottled water in modern country while isn't necessary, is a precaution worth considering.
I do both, in countries I am comfortable with (England, Switzerland for example), I often drink tap water, but many countries I stick to bottled water.
I have suffered amoebic dysentery twice, hope never to do that again.