Here's how the Active Denial System works.
It uses a large, dish-shaped antenna and a long, V-shaped arm to send an invisible beam of millimeter waves to a target as far away as 500 yards.
The beam penetrates the skin only slightly, but just enough to cause intense pain as vibrating molecules create high heat.
Water weapon, more han a squirt gun
Using water as a weapon has several benefits: it produces no shrapnel; it reduces collateral damage; it can short out electronics; and it's an abundant and free resource.
Police and military use robots armed with a "water jet disruptor" when dealing with potentially explosive devices. The disruptor uses air pressure to shoot water through the wall of the explosive, either shorting out the electronics or detonating the explosive.