What sensor would you use to track how much liquid is left in a bottle?

Using Arduino Sensors
6

Arduino420:
Or this one to work in direct contact with the water itself.

http://www.ebay.com/itm/Water-level-sensor-liquid-level-Water-depth-detection-High-Sensitivity-Arduino-/251191540408?pt=LH_DefaultDomain_0&hash=item3a7c2ebab8

That seems to have a pretty restrictive temperature range?

10.Operating Temperature: 10 ? -30 ?