Cashiers at my store used to have water bottles at their registers, but all of a sudden the ETLs and TLs took a hard line (no pun intended) on it and disallowed it. We had 3 cashiers quit that week, for that reason.
If I wanted to, I could probably get around this issue. My TLs and ETLs are enough of group of pushovers that I could tell them I have epilepsy (which is not a joke, I really do have "primarily generalized epilepsy," according to my neurologist) and insist that I be allowed to carry a drink on the floor. All I would have to tell them is that low blood sugar/fatigue is one of my seizure triggers, and they probably wouldn't push the issue.