"A slaver looking to kidnap human cargo from small tribes of the West Indies will soon be meeting with other slavers on Andreas Island. Intercept him and put an end to his plans."
Head to the Restricted Area surrounding the dock. You will not find your target in there immediately. Instead you will see a ship coming into port. If you wait for the incoming ship then you will find your target. Take out the ship's guards on the docks with Sleep Darts if you want to do a more showy finish on this one. Just wait for the target's ship to come into range and take them out with a Dart or pistol shot.
You can also just sink the ship with the Jackdaw and be done with it.