This mission unlocks after you take over the Longshore View outpost.
Get to the area marked with the blue exclamation mark and accept the mission of avenging the murdered family. Go to the first encampment and kill an enemy marked with a star, after which you should take cover in the river to the right (relative to the screenshot). This way you will avoid fighting the remaining enemies.
Swim up the river to approach the next objective of the mission. Once there, you will notice another enemy standing on a boat. If you have the skill Death from Below available, Swim up to him quietly and kill him. Then, dive again and start swimming towards the next target, to escape from the remaining enemies.
Once you reach the area, which is supposed to be occupied by the last target, get out of water near the pier shown in the screenshot.
Climb the stairs and walk over to the right of the building. That is where the last one of the family's murderers should be. After you kill him, return to Joseph to complete the mission.