Reach the captain's cabin in AC4 Black Flag

"Reach the captain's cabin" (Buy Broadside Cannons) is a mission objective in Sequence 3 / Memory 04: Raise the Black Flag


After bribing the Local Officer at Salt Key Bank, swim back to the Jackdaw and go to the Captain's Cabin by entering the highlighted doors under where the ship's wheel is

There, you should buy the Broadside Cannons