Robert Deal wants you to come see him. Said he has some work for a true Buccaneer.
At Nassau, there are six hostile ships ranging in level from 3 to 8. You get one such ally. Your ally and the merchant ship you must board start quite near you, while the other ships start far enough away to give you plenty of time to fight the merchant ship alone.
After you defeat the merchant ship in boarding combat, you must reach the escape point, which is quite a distance from where you start. Sail straight there and the enemy ships will mostly go after your ally and leave you alone.
Return to Robert Deal in Morgan's Bluff to complete the mission.