It’s, “A boatswain (/ˈboʊsən/, formerly and dialectally also /ˈboʊtsweɪn/), bo’s'n, bos’n, or bosun, is the senior crewman of the deck department and is responsible for the components of a ship’s hull. The boatswain supervises the other members of the ship’s deck department, and typically is not a watchstander, except on vessels with small crews. Other duties vary depending on the type of ship, her crewing, and other factors.”

Blame Wikipedia – http://en.wikipedia.org/wiki/Boatswain