Coxswain and Boatswain

As required by 17th Century law, British ships-of-war carried three smaller boats: the boat, the cock-boat, and the skiff.  The boat, also called the gig, was usually used by the Captain to go ashore and was the larger of the three.  The cock-boat was a very small rowboat used as the ship's tender. The skiff was a lightweight all-purpose vessel, generally with a flat bottom. The suffix "swain" means keeper, thus the keepers of the boat, cock, and skiff were called boatswain and cockswain (or coxswain).
Today, any boat used by a ship's Captain is referred to as the gig.  Embarked Admirals use a boat referred to as the barge.  Skiffs are generally aluminum and are used for painting the sides of the ship.
In common use, boatswain is usually shortened to bo'sun.  A ship's bo'sun is the senior member of the deck hands.  The coxswain mans the helm (rudder) of a ship's boat when underway.

A coxswain or cockswain was at first the swain (boy servant) in charge of the small cock or cockboat that was kept aboard for the ship's captain and which was used to row him to and from the ship. The term has been in use in England dating back to at least 1463. With the passing of time the coxswain became the helmsman of any boat, regardless of size. 

Another reference defines “Swain” or Swein” as Anglo-Saxon for servant. Boatswain refers to the warrant or petty officer in charge of the deck crew.