You're forgetting the people who absolutely were not fine. If those people die in childhood often enough, that eventually gets removed from or significantly reduced in the gene pool, but people have to die first. It's like asthma. Those kids used to just die. Now we have inhalers, so it appears like asthma is on the rise when it's more that less people die from it. It looks like adults who grow up drinking that water are fine, but you're ignoring the toddlers who died from drinking that water.