BOATSWAIN. The warrant officer who in the old Navy was responsible for all the gear that set the ship in motion and all the tackle that kept her at rest. [Sir Geoffrey Callender, "Sea Passages," 1943]
He also summons the hands to their duties with a silver whistle. Phonetic spelling bo'sun/bosun is attested from 1840. Fowler  writes, "The nautical pronunciation (bō'sn) has become so general that to avoid it is more affected than to use it."