From the start, you will see a pair of wrecked ships. There is a chest in each of them. Quickly swim through the wreck on the right, grabbing the first chest and dodging jellyfish as you go, then swim down through the gap in the top of the wreck on the left for the second chest. Pop up and use the air barrel nearby.
From the air barrel, you will see an underwater cave entrance up ahead. After refuelling your air, enter the tunnel and follow it to the right. Upon entering the first room you’ll see an Animus Fragment behind a wall ahead, but we can’t grab it just yet.
Follow the tunnel to the right until you reach the next room. At this point, swim to the left to find the tunnel leading to the Animus Fragment. Return to the previous room and continue through the tunnel until you are back outside.
Use the air barrel here to refill and then look below it to find a ship hull containing a chest (avoid the eel!). Follow the ship through and out the far end. Swim up to the deck of the next wreck ahead and use the air barrel here if you require it. Dodge the jellyfish and Interact with the glowing hatch on the deck to open it and swim inside.
Grab the chest from the floor here and enter the next tunnel. Swim through until you reach the first air pocket. Grab the chest on the ground here and then surface to replenish your air. Dive back down and follow the jellyfish tunnel behind the chest. As you go, keep your eyes peeled for a small alcove on the left containing an Animus Fragment. Once you have that, keep going until you reach the next air pocket.
At this point, we can climb out of the water. Climb the ramp and loot the pair of chests here. Afterwards, dive down the hole to reach the next water source. Here immediately duck underwater and grab the chest here.
Enter the tunnel and you will be propelled forward by water currents. We have limited control at this time, so do your best to dodge the obstacles in your way. Eventually you will reach a tunnel on the far side of the area. Move forward down the tunnel a short way to find an air pocket. Swim up to the surface in here to regain some oxygen.
Afterwards continue through the tunnel a short way and you’ll be back outside near the diving bell. Interact with this to return to the Jackdaw.