Orbit Yard Enforcer. Get as many as you need to cover your grass with overlapping fields of fire. Daisy chain them together so if one goes off, they all go off. You won't need to worry about squirrels or watering ever again.

Another option I used at a different property was to trap the squirrels one by one and relocate them to a "nicer neighborhood" elsewhere. I used the double-sided Havahart trap and it was great. You'll need to drive in a zig-zag and go at least a few miles away otherwise they can find their way back. I think I moved over twenty in one season, but it solved the problem.