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).
Sam2:
Interesting. If he really holds bottled water is such an issue, why would Rav Schachter as the senior Kashrut consultant for the OU allow the following to be published in their 2013 Pesach Guide? (p. 91):
All unflavored bottled water is kosher for Passover, even without
any kosher supervision