Why not just rig up a small sprinkler system that you can turn on or off . You get to water the garden effectively in the summer and the added bonus of soaking the cats when you get the chance. I use a couple of more basic methods
1) I corner the little gits in my garden using the hose pipe and drench them
2) I entice them in with a couple of strokes then get a glass of water straight in the face while being held by me.
Both seem to be very effective
My 20yr old cat is now past patrolling the garden so she just sits there and watches these days