Park outside in the boat and enter the cave. Follow the left-hand wall until you find yourself on some platforms. Hop across these until you see a Gunner up on an upper level platform ahead on the left. Work your way over to this location and climb up to the platform with the Gunner – be sure to grab the Animus Fragment as you climb.
Drop down from the platform here and dispatch the four guards nearby practicing their shooting. There is a chest inside a tent in this main camp area. Climb up onto the fence behind where the group of enemies were shooting and from here, air assassinate the guy by the campfire below. Cross the bridge and kill the guy around the corner to the left with a sprint assassination. Loot the chest here.
Drop down to the dockside area below. Go over to the far west side of this platform to find a cadaver with a treasure map (70, 405 - Tulum). Just east of this is a third treasure chest. From this chest, look directly north, you should see an Animus Fragment on a small wooden deck.
Now dive into the water and swim past the back of the ship to the north. Here you’ll find a rowboat, and in an alcove in the wall just to the west of it – the final chest. Hop in the rowboat to return to the Jackdaw.