An ancient system of trial by ordeal sometimes also used for punishment dating to ancient Babylon, where it was used in cases of alleged adultery. It was made most famous during the middles ages in Europe where it was used to punish “scolds” (a woman who would break the peace by frequently arguing with neighbors) or to prove whether an accused person was a Witch. The technique could be as simple as tying a person’s hands and feet and throwing them in water. A more complex version involved tying a person to a chair suspended over water by a long pole with a counterbalance so the person could easily be dunked into the water. The dunking was repeated until the person either floated or drowned.
Depending on the locale, a person was judged to be a Witch if they floated (because water, the instrument of baptism, rejected them) or sank (because water enacted God's judgment and punishment).
The submersion of the head in water is generally regarded as a form of torture. It is currently debated as to whether a form of this, known as “waterboarding,” is actually torture.