Now that was a fun one to watch but meer water wouldn't do the trick, if going to the trouble of stopping cats the best way would just use a 555 based circuit and a car coil. Lay a series of wires down in the cats path then when the PIR is tripped 20,000 volts and that would be a classic to watch..... especially when it landed on the wires again.......
Cruel me NOWAY a bit twisted I suppose.......