Bad idea. Stupid idea. Ok, only cops should be required to use this. It _might_ save their lives if a perp snatches their gun. But, of course, as the cop is trying to snatch the gun back, the watch will probably be in sufficient proximity to the gun to activate it.
What if you are left handed and wear your watch on your non shooting hand?
No biggy. They'll just come up with a module that gun owners will be required to have surgically implanted in their anuses.
Changing the batteries might be a pain, but it will be right in the middle of everything, so no more worrying about which hand to wear the watch on.