Ducking and shaving
Ducking and shaving was the practice of shaving new seamen and ducking them in the sea the first time they cross the Tropic of Cancer. The custom, common on 18th-century British sailing vessels, involved tying a rope around the man and dunking him in the sea three times from the main yard. Sailors could pay a fine to escape the rite of passage, but most accepted it, many enthusiastically.
At other times, ducking was used as a punishment similar to keelhauling.
A Slaving Voyage to Africa and Jamaica: The log of the Sandown, 1793-1794, edited by Bruce L Mouser, Indiana University Press, 2002, page 31