E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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.