Upon accepting this contract, a red skull will appear on the map off the coast in Annoto Bay (just to the east of Long Bay). Sail over here and dock with the shipwreck to dive underwater.
Once you are down, continue straight forward and enter the cave here. As you enter you’ll note that it looks kind of like a mine shaft. Continue forward until you see a ladder, swim up to the top of this. Hang out here for a few moments and wait for a shark to swim by.
Follow this and continue to the end of the hallway. Turn left here and then right when you reach the next wall to find a ladder. Swim up the shaft here and quickly turn around to find a chest. Now swim around the corner to find an air pocket. Surface here to replenish your air supply.
Dive back down and follow the hallway to the end, turn left to find an Animus Shard. Now follow the tunnel to the right. Swim up to the top of the ladder here to exit on to dry land run around the corner and dive underwater again. Now as you swim through the tunnels, hug the left hand wall and follow the area through until you find another ladder. Surface above this to find a smuggler’s hideout.
Now loot the area. There is a treasure chest just in front of the ladder on the north side of the room and another by the top of the stairs leading into the water on the south side of the room. On a walkway on the eastern side of the room is a cadaver with a treasure map. In the water in the centre of the area, if you dive below you’ll find a tunnel. Follow this to the left and surface when you are able to find an Animus shard.
Climb up onto the ledges above the water here and hang out for a moment. Use Eagle Vision to find the assassination target. Now simply use the ledges to take everyone down undetected using ledge takedowns and stealth swimming (we need to be undetected for the 500 Reales bonus) or just march in and murder the lot of them to complete the contract.
When you have everything, climb the ladder on the north side of the room and dive into the ocean below. Climb back onto the Jackdaw to finish up.