| Before heading to this location it is worth noting that it has a Treasure Map that can be obtained from Santanillas. |
From the diving bell swim into the tunnels and follow the linear path as you avoid sea urchin and moray eels. As you approach the obvious Animus Fragment make sure you swim over the eel to grab it and then surface in the air pocket for some air. As you swim west through the tunnel you will get caught in a short current that will drop you by a Treasure and then continue through the tunnels until you reach the smuggler's den.
Start by ledge assassinating the first guard and then kill his buddy to get the second Treasure. If you have the Treasure Map for San Juan you can find the dig location on the ledge to the right of the first guard's location. Climb the ladder to the next area and air assassinate the one guard against the wall while the patrolling guard isn't looking and then take out the patrol. Leave the rest of the smuggler's alone, as they are in pairs and don't pose a threat, and then loot the Treasure. Climb up and deal with the Marksmen before grabbing the Animus Fragment and leaving.
