Assassin's Creed 4: Black Flag Walkthrough Part 78 Shipwrecks & Caves: “Ambergris Key”

It is best to do this dive after you get the map from the nearby Corozal and the Assassination Contract “Smuggler's Den” to avoid repeat trips here for things. Be sure that you have either Rope Darts or a throwing knife in your inventory to make some things easier. With those in hand, start the dive.

Once at depth you will see a number of Jellyfish blocking the passage. They can be navigated but it does take a moment. Just move through carefully and get into the cavern in front of you. Head on through to the air pocket. Before you surface, take a moment to go along the bottom and collect the first of 3 chests. Now surface, avoiding the jellyfish then dive back down.

Continue on through the passage to the next chamber. It is very straight forward swimming. All you need to do is be mindful of the jellyfish and the sea urchins. When you surface again, move forward carefully. Keep to and head to the very southern portion. Investigate the small beach there to find a chest with 4,000 Reales inside it. Now head to the North. Climb on up the pillars beneath the ruins to find an Animus Fragment. Now go over to the West and duck into the cover of the ruins there. Wait for the patrolling guard to move away. After that use the Throwing Dagger to kill the stationary guard. Now climb up to where he was. Either take a corner position or grab the nearby musket. Kill the patrolling guard then move toward the next chamber.

In the next chamber you can identify your target without much trouble to the right.  He will come close enough that you can lure him into a corner assassination. Now just fight and kill off all the other Soldiers, Brutes and Captains in the area. Now start around the area to claim the final 2 chests. Be sure to also claim the treasure map. This will lead to Mistereriosa where you can claim the Elite Heavy Shot plans (fast travel to the view point to quickly reach them).

The final matter here is the second Animus Fragment. It is somewhat tricky to obtain. Head back to the doorway you entered the ruins from the underwater cavern. Climb straight up the rubble in front of you and go straight through to find the second Animus Fragment. Now drop down to the ground and head to the ramp in the Southwest. This will let you dive back into the sea and get back to the Jackdaw to end the trip. Just take the wheel to have everything snap into place.