Jacob: Rav Schachter explains quite clearly why, Al Pi Din, there are several reasons that bottled water needs a Hechsher. The first and most obvious is that the bottle might have Tarfus in it (Kavush Kim’vushal).
I want to make something very clear in this thread. There is a difference between the type of Machmir that Rav Blumenkrantz was and types of Chumros in certain other Seforim (I’m not judging any individual other Sefer here; just commenting about R; Blumenkrantz). Rav Blumentkrantz was a major Posek that explained the Halachah Ad Buryo and was just often Noteh L’hachmir. That is fine. We have Poskim that are more Noteh some ways than others. It happens. It might mean that we don’t hold like Rav Blumenkrantz all the time (as he is Machmir more often than the average Posek and therefore it makes sense that there are cases where we accept his P’sak and cases where we don’t). He does not misrepresent Chumros as Halachos. He does not take personal Chumros or safeguards and impose them upon the Tzibbor. Calling him a Machmir as a dismissive term is offensive and lacks Kavod HaTorah. Saying he is more Machmir than average and therefore we don’t always hold like him might be okay (if done properly). Dismissing him as a “Machmir” is just wrong.