When I was in Cuba, I brushed my teeth and rinsed my toothbrush with bottled water, and I was glad that I did.
For much of the time, I was eating in tourist-oriented restaurants in central Havana, but our group's farewell dinner was in a less upscale restaurant, and I ordered a mojito.
Evidently, the central restaurants made their ice with purified water and this last place didn't. The woman who sat next to me ordered the same food that I did but had beer as her beverage.
I got sick (sudden onset vomiting for 24 hours), and she didn't.
Lesson learned.
In China, I was scrupulous about avoiding tap water after learning (before the trip) that Chinese people don't drink unboiled water. We mostly stayed at school dormitories, and every morning, we'd receive a thermos of boiling hot water. We took to pouring some off into the covered teacups that were in the rooms and then using the cooled water in the evening to brush our teeth.
In Western Europe and Japan I don't worry about the tap water.