Note: this is an underwater location which requires a Diving Bell to access.
You have the Treasure Map that leads here as well as the Assassination Contract The Smuggler's Squat. When you have both these things, then it is the best time to visit the location.
From the Diving Bell start, swim to the opening to the South of there. Travel through the submerged mine tunnel until you find a ladder. Follow it up. Pause at the junction ahead to let the shark swim on ahead of you. This shark swims a counter clockwise circuit through the area. You need to press into its territory to refill your breath and to open the chest int he same area. As soon as it passes by then take the tunnel to the left. Down this way you will find an air pocket. Return to the corridor with a fresh lungful of air. Wait for the shark to go by again Turn left and swim to the South until you reach another ladder. Swim on upwards. When you get to the top you will find a chest in the dead end to the east. Make a u-turn after that and head to the Air Pocket to the Southwest. Continue through to the South until you reach a junction. Turn left there to find an Animus Fragment. Once again, make a U-turn and swim West until you reach a ladder. Lunge up to the surface and climb upwards. There is one more swim to get to the Smugglers but it is very straight forward.
Dive into the water and go right at the first intersection. Follow this corridor a while. Turn to the left when you can, twice. be sure to use the lunge bar to speed up Edward's progress after the second left. Head to the end of the corridor and climb on up.
Once on the surface head over to the dock. Take out the nearby Smuggler with a ledge assassination. Follow that by climbing onto the dock and attacking the other smuggler who is attacking the prisoner. Turn to the East and head into the central area. This leaves 5 hostiles in the area. Engage them in a brawl to just finish them all off. The two remaining chests and treasure map are easy enough to get. It is the Animus Fragment that takes some doing.
The Animus Fragment is found by diving into a hole in the floor in the Northern portion of the room. Follow it to the West. Swim to the end of the tunnel to reach the corridor with the Fragment.
The treasure, from the map for this location, is found just a few steps away from the ladder leading up to the cave's exit.