I hate to break it to you all, but there is one potable water tank on the 320's and 737NG's. This tank feeds all toilets, sinks, and coffee makers. I don't think any mainline aircraft have the older re-circulation "blue juice" toilets.

On the 320's, the indication panel is in the front galley. 737's its in the rear. Yes, it would be a FA's pre-flight check. That said, it is unlikely this went overlooked. When the water/waste system is serviced before each flight, the contractor calls it in to operations. If they do not enter this into the computer, it locks the flight crew from receiving a final weights manifest. (a stop gap measure for this not happening)

More then likely, the inoperative water was due to a maintenance issue, and the FA's kept it simple.

found this courtesy of Aeroflot. gives you an idea of the airbus.
http://lms.aeroflot.ru/data/U0FT1M0/...0&%20WASTE.pdf