“Live ammo would be much more effective.”
My first thought also but then I revised my thought to rubber bullets. They sting like heck and occasionally they hit the right spot and an invader starts their dirt nap. In Texas using live ammo would result in polluting the river with decomposing bodies floating down stream making the water unsuitable for livestock.
Paint balls, pepper spray balls.
Lots of effective deterrents out there to chose from.
They could add a chemical to the water canon spray to make it smell bad or cause nausea.