The title of Captain is the title given to the person in charge of a ship at sea, either pirate, marine, or civilian.
The rank of a marine officer who commands a ship would not necessarily be "Captain" (大佐,Taisa?)", it may be higher (up to Fleet Admiral), or, on small ships, lower. Any officer below the rank of Taisa who commands a ship, is addressed as "captain" while aboard that ship, by naval custom. The ship's captain also may be referred to as the Skipper.
Role and Duties
Pirate Captains are the ones who are at the very head of a Pirate crew. Pirating captains also tend to be the mightiest members of their crew, and most of them tend to exploit their strength even over their own subordinates to ensure loyalty; though not necessarily, as anyone can attain the rank of captaincy out of pure leadership, as a founding member or voted by other crew members. All major decisions are made by them.
In real life, captains of pirate crews were often elected to their posts by a ship's crew. Even if they owned the ship, if the captain fell out of favor with their crew, another could be elected without notice. Sometimes, inadequate captains were even murdered.