A lot of the advice being offered right now is standard all-occasions preparedness, which includes water in case of natural disasters. But as someone pointed out up-thread, most municipalities keep their staffing levels at the bare minimum due to budget pressures so in smaller communities it wouldn't take too many people to fall ill to interrupt ordinary operations - my community's water authority employs 3 full time and 2 part time workers, and only the 3 full-timers have a direct hand in the processing plants (the part time jobs are clerical). We do have a back-up plan of calling in a few department retirees that still live in the community should they be needed, but that's still only 5 or 6 people that are charged with keeping the drinking water system for a couple thousand households running.
That said, I'm not concerned enough about that eventuality to stockpile water. I think disruptions to the water supply are much, much less likely than circumstances where I might not want to have to go to the store for a couple weeks.