Something engineers, mainly Machinist Mates, do in the U.S. Navy to each other in the boiler/engineering room. A container of water is dumped from an upper level area to someone on the lower level, or from any high area to any lower area. It is always meant to be a surprise. It is also done to people who are sleeping on watch as a way to wake them up, and they do so in shock, to teach them to not sleep on watch or else it will happen again. It is also done to people who are already awake as a practical joke. Something like that would usually never be done to any higher-ups, especially those that don't participate in the game, since it will get you in trouble.
Petty Officer Clark: You see Price sleeping in the sscape exit? Let's water hammer him!
Petty Officer Price: I just got water hammered. Do you know who did it?
Fireman Stovell: Yes, it was Clark.
Petty Officer Price: I'm gonna water hammer him next chance I get! It's on now!