Wow,
There are so many options!
If you need great reliability go with a comercial sensor (and there is a bewildering array of them) Check out McMaster.com
If you are making your own consider these ideas.
-A Sharp IR distance sensor looking down to the liquid
-same Idea with a sonar sensor
-A float with a magnet and a hall effect sensor (sensors can be found with open collector and ttl outputs)
-Not green but you could find an old thermostat with a Mercury switch and put it on a tilting float
-Lots of circuts that use the conductivity and/or capacitance of the water are available on the internet.
-if you are actually using a 2 liter bottle you could arrange opto sensors to look through the bottle and note the change in transparency.
Figure out your needs and take a pick