This is not a guide but more a tip for people who struggled to unlock this at first since I wasnt sure what was required to unlock it. I had finished the dlc main quest, had done all collectibles, speed trials etc. Still no achivement. Was the last achivement I had left to unlock. Even done the quest back in main world for killing sailors in different regions. Only thing I had left on map was these mission to ”destroy boat” or kill this or that ”iradian Shark”. Thought it was only for boosting your xp or unlocking stuff since I already beat all these sharks/bosses.
But then ran before thinking it was glitched I thought I go around and clear all obj on the map, even thought truth quest objectives were all done. And after finishing everything there was, both main game and dlc, Every marker/mission. It popped!