Cold Water Ordeal
An ancient method of testing the guilt or innocence of the common sort of people. The accused, being tied under the arms, was thrown into a river. If he sank to the bottom, he was held to be guiltless, and drawn up by the cord; but if he floated, the water rejected him, because of his guilt.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894