A good water flow meter to monitor home usage with Arduino
I've used an arduino with an irrigation flow meter. 

I think I'm getting accurate results, but I haven't checked against a bill. It can handle high flow, and the sensors will attach to any size pvc line. The sensor are a bit expensive, however. $150 for the sensor and $35 for the T junction that it plugs into. 

However, as you are living in an apartment, you'll probably need a non-invasive sensor. Perhaps an acoustic one.