Personally I have had god luck detecting a high water level with the digital IO method.
I have the PIC pin pulled low and apply a positive voltage to the probe. No current is used untill the water makes contact. When contact happens you will want to turn the power off to the probe to save power and to cut down on oxidation on the probe.

You can use a timer off of the PIC to periodically turn the power on to the probes and check for conductivity.

What kind of water is this? Potable, seawater, sewer? Decide what to use for the probe material? If it is sewer watch for sparks