This argument is extremely naive and infantile. There's absolutely nothing strange about a culture finding out from experience, and deduction, that dirty water is bad for one's health, while clean water is better. Why would anybody think this shows some kind of impossible knowledge from divine inspiration? It's not even any kind of marvelous feat of knowledge or deduction, but something quite trivial.
However, the argument falls really flat when we examine what Jesus had to say about the subject. In Mark 7 we have this story:
The Pharisees and some of the teachers of the law who had come from Jerusalem gathered around Jesus 2 and saw some of his disciples eating food with hands that were defiled, that is, unwashed. 3 (The Pharisees and all the Jews do not eat unless they give their hands a ceremonial washing, holding to the tradition of the elders. 4 When they come from the marketplace they do not eat unless they wash. And they observe many other traditions, such as the washing of cups, pitchers and kettles.)What a wonderful opportunity for Jesus, the son of God, to demonstrate supernatural divinely inspired knowledge about microbes and the reason why washing your hands before eating is good for you. Instead, Jesus criticizes the Pharisees for following that custom.
5 So the Pharisees and teachers of the law asked Jesus, “Why don’t your disciples live according to the tradition of the elders instead of eating their food with defiled hands?”
Mind you, a custom that according to these Christian apologists, is divinely inspired and perfect.
Jesus concludes his criticism with this:
14 Again Jesus called the crowd to him and said, “Listen to me, everyone, and understand this. 15 Nothing outside a person can defile them by going into them. Rather, it is what comes out of a person that defiles them.”Again, this is a missed opportunity to explain why dirty hands and dirty water cause disease. Instead, Jesus criticizes the Pharisees for following such rituals.