The boatswain supervises the other members of the ship's deck department, and typically is not a watchstander, except on vessels with small crews.

It is derived from late Old English batswegen, from bat (boat) concatenated with Old Norse sveinn (swain), meaning a young man, apprentice, a follower, retainer or servant.

[3] The boatswain was the officer responsible for the care of the rigging, cordage, anchors, sails, boats, flags and other stores.

It is equivalent to the rank of colour sergeant in the army and the royal marines cadets; it is sometimes an appointment for a senior petty officer to assist a coxswain.

[4] A boatswain must be highly skilled in all matters of marlinespike seamanship required for working on deck of a seagoing vessel.

These duties can include cleaning, painting, and maintaining the vessel's hull, superstructure and deck equipment as well as executing a formal preventive maintenance program.

The boatswain is well versed in the care and handling of lines, and has knowledge of knots, hitches, bends, whipping, and splices as needed to perform tasks such as mooring a vessel.

Effective boatswains are able to integrate their seafarer skills into supervising and communicating with members of deck crew with often diverse backgrounds.

During World War II Bosun John Crisp RN is credited in "The Colditz Story" by escapee Pat Reid as providing, whilst a prisoner of war at Oflag IV-C, Colditz Castle, the expertise and enthusiasm to manufacture torn and then woven "bedsheet ropes", tested for appropriate strength, using his extensive maritime experience.

Typhoon by Joseph Conrad has a nameless boatswain who tells Captain MacWhirr of a "lump" of men going overboard during the peak of the storm.

[14] The 1907 naval gothic novel The Boats of the "Glen Carrig" by William Hope Hodgson features the character of the ship's “bo'sun” as an important member of the crew and a personal friend to the narrator.