Keelhauled

Keelhauling (Dutch kielhalen "to drag along the keel") is a form of punishment meted out to sailors at sea. The sailor was tied to a line that is looped beneath the vessel, thrown overboard on one side of the ship, and dragged under the ship's keel, either from one side of the ship to the other, or the length of the ship (from bow to stern).
The common supposition is that keelhauling amounted to a sentence of either death by extreme torture, or minimally a physical trauma likely to permanently maim. The hull of the ship was usually covered in barnacles and other marine growth, and thus keelhauling would typically result in serious cuts, loss of limbs and even decapitation.


If the victim was dragged slowly, his weight might lower him sufficiently to miss the barnacles, but this method would frequently result in his drowning. There was also a risk of head trauma, especially if the ship was in motion.

https://en.wikipedia.org/wiki/Keelhauling