Me (freshly arrived in Korea for the first time): "Can I drink the tap water?"
He (American, several years in Korea): [Calmly] "Never drink the tap water."
That was that. I didn't question it. Why would I? He was the expert; I was a complete outsider.
Many Westerners who end up in Korea don't get the message. I've known a few who openly say they drink tap water. Never have I heard a Korean say this. I most recently met a tall young man from Georgia (born 1993), recently arrived in Korea, who triumphantly announced that he drinks the tap water. My first instinct at his proclamation was that he needs to get with the program and do as the local people do. Then again, maybe the local people are all wrong and he should do what's right after all. (The Herald itself says that the people are wrong and the Korean tap water is fine.)
What does it say about Koreans that they drink their own tap water at such low rates compared to the rest of the rich world, the USA and other Western countries (e.g., Germany: 90% drink tap water according to the Internet consensus)? The easiest answer is that Korea was poor just a short time ago and so blah blah blah. This is a stock answer to a lot of "questions about Korea". More interesting lines of speculation are possible. Here is one: The desire for elaborate filtration systems or bottled water comes from the same place as the custom to always take off shoes at the door at all times. I think you may see where I'm going with that. Another possible speculation: For all Korea's wealth today and its impressive "soft power" in Asia, Koreans don't necessarily trust their own institutions (water as a social microcosm).