Water Sensing Circuit
You could use this circuit instead. The capacitor on its input will filter out most interference and the PNP transistor will give better output voltage levels.

If it's still too sensitive to interference, increasing the value of C1 and/or R3 will help. If the water doesn't trigger it reliably, increase the value of R2. (The circuit will work for a water resistance of about 15MOhm in its current form)







simulate this circuit &ndash; Schematic created using CircuitLab

As with any circuit that puts a DC voltage across water, the electrodes will slowly corrode when submerged in water.

You could also design a circuit that measures the capacitance between two twisted, insulated wires sticking into the water, but that would need an oscillator and time measuring circuit. It's possible to implement this in the PIC but it would be way more complex and might also need more power. The main advantage of such a circuit is that it's even less sensitive to (conducted) interference and doesn't corrode the electrodes.