How about keelhauling?
When a sailor was keelhauled, he would be stripped and tied so that he could not swim. Usually, a weight was attached to his legs to pull him away from the ship. The sailor was attached to a rope that ran underwater from one side of the ship to the other, and he was rapidly pulled through the water. Assuming the sailor did not usually drown, he would severely injured by the extremely sharp barnacles on the underside of the ship, known as the keel. This practice would leave severe scars on the flesh of the sailor, serving as a constant reminder of the event.