If you want an accurate measurement, you can put a tube that exits below the lowest water level you expect. Put a pressure sensor like an MPX2102 and an aquarium pump on the tube. When you turn on the pump and it starts bubbling out the tube at the bottom, the pressure you measure on the sensor will be relative to the height of the water column above the tube opening. 1"wc is approximately 249.082 pascals or 0.0361263 psi

I use this method on my brewbot for measuring the amount of water in the kettles for strike and sparging. It's accurate to within a 100mL or so, at least with the ADC I'm using on my controller. You might need a slightly different sensor depending on the over height of water column you plan to measure.