If you wish to play, here is the circuit I'm currently using:
Attached is the Arduino code I've been running (LevelSensor.txt). Every 500ms it will send out the serial port the RC time count.
You can even use a Solo cup to test with: Wrap about 1/2 the cup with standard foil. Run a ground wire inside the cup and make sure it hits the bottom (I use a small nail to weigh down the wire) and connect the foil to the PD6/Digital pin 6 end of the 10M resistor.
When running you'll see numeric values on the terminal. As you fill the cup with water the values will increase (and if you somehow empty the cup without disturbing your test setup you'll see them fall).
The AIN1 voltage divider provides the 4.5V reference for the comparator to capture charging the RC tank cap. The ADC0 provides the 0.5V reference for the comparator to capture the discharge of the RC tank cap.