We're gonna be pirates, and be all piratey.
Okay, well we know the typical two ranks we see in movies
However, the 'crew' would be very broad on terms of duties, and the captain would have a flux of responsibility.
Captain (Ultimate authority, must have an ability to win prize and booty and must be daring, where the ship might sail, or rather or not to put into port, or if a certain ship should be engaged and so on. although, crews have been known the maroon/throw him overboard if they disagree with his judgment)
Quartermaster (Effectively the second in command, distributes the punishment, rations, work and what not. Is often raised to Captain if anything happen to the captain in command such as being marooned/thrown overboard)
-Thugg (Next inline for Captain)
Boatswain (Basically is the carpenter, in charge of repairs to the ship, when making port it's up to the Boatswain to deal with matters regarding cargo - which was originally decided by crew/captain - typical place in command under the Quartermaster; deals with the flag and ship colors being properly maintained/created. Also in charge of stealing other ships, and random assassinations.)
Privateer (People who shoot the cannons and keep the weapons in order)
Pilot (Navigator, steers the ship)
Striker (They hunt the food, fish and turtles and such)
Doctor (Not necessary, but you know people can get sick/hurt..)
Ordinary seamen (Do whatever no one else wants/feels like doing. Scrubbing the decks, raising the sails, bucketing out the water that the ship takes in, and what ever else.)
So What do you want to be? :)