They are descended from the old dunk tanks used as punishment in the Middle Ages, where people accused of witchcraft or other "crimes" would be tied to a chair and dunked underwater.
In modern dunking booths, you just get wet.
The "victim" sits on a level platform suspended above a tank of water. People line up to try a feat, like hitting a target. If they are successful, the platform falls away, and he falls into the water.
Sometimes there's no feat; you pay for the privilege of pushing a lever and dunking the victim.
The tank is often made of clear plastic so you can watch him flailing around.
The victim is usually male, often a teacher, preacher, camp counselor, college football star, or other high-prestige figure, to make the dunking a kind of "revenge."
But don't worry: Dunking only occurs on hot days, when being dunked repeatedly is rather refreshing.
And you get to see a lot of attractive men in short pants or swimsuits.
See also: Celtic Festivals