There are OSHA regulations around this. One time the office building I was in lost water and w/in an hour or two, they had a fleet of port-a-potties and hand washing stations out by the delivery bay!
They don't necessarily *have* to let you go home. Is your entire building without water? Are there other buildings close by with water?
Actually, they do if the building is without running water for a certain amount of time (a couple of hours I believe). The only time in that 10 years my office has officially closed was during a hurricane where we didn't have running water in the building. The police showed up and told them they had to close and escorted everyone out (everyone being the office manager, the hourly employees and admin staff that don't have the option to work from home, and a few of us that had a proposal still due that day).
Not sure what other buildings around having water would have to do with it.
Is this a tall building? I ask because the fire department would make you evacuate the building if there is no water. We had a water main burst at my old job and they evacuated the whole building--like get out NOW-because if there had been a fire, they wouldn't have been able to fight it.