Assassin's Creed 4: Black Flag Walkthrough Part 80 Shipwrecks & Caves: “The Blue Hole”
The first chest is a little complex to reach because of the 2 sharks in the area. Start by leaving the Diving Bell and going over to the left of the hull. Duck into the patch of seaweed there. Next make for the covered area of the deck when the shark moves away again. Finally make for the chest itself which is just below you through an opening. Now make for the nearby air barrel. Look downward from there to find your next destination: a chest. Open it then turn to the East and go through the nearby doorway. Go in and over to the left. Wait at the opening for the shark to just start moving away from you. After that go for the captain's quarters and open the third chest of the dive. Now turn to the left and swim over the side of the ship. Toward the stern of the ship you will find a hole leading inside the hull. Go straight in to find an Animus Fragment. Now backtrack to where you entered the hull and swim down. There you will find the fourth chest. Now swim out of the wreck and to the North. There is an air barrel nearby for you.
From the air barrel, go to the West. There is a chest with a Moray Eel nearby it. Chance getting to it over the rocks and you will be sure to avoid the Eel. After that start back to the West. Go over the rocks in that direction then make your way Northeast over more. There you will find a treasure chest with the “Elite Broadside Design Plans” inside them. Now return to the patch of seaweed on the other side of the rocks.
There is a very oddly placed Animus Fragment for you to collect. Swim up to it carefully, being mindful of the nearby shark. Grab it then head down to the nearby air barrel. From that air barrel make your way Southwest through the next few seaweed patches. After that you will be closer to the final chest. When it is directly to the South of you, then start swimming toward it. There are 2 Moray Eels and a shark nearby to worry about. For the Morays, keep low to the ground as both have some height to them. The shark is not a concern until after this chest. Grab the chest then go to the nearby air barrel.
Time to claim the final Animus Fragment then get out of here. Swim carefully out toward the final Fragment. Keep close to the hull of the ship. As you approach the darkened piece of wreckage, dive low. Swim into it from below, grab the Animus Fragment then swim out and start off to the West. Keep at it for a few seconds then turn to the South and make for the Diving Bell when you line up with it. It can be close in terms of breath but Edward can make it.