Assassin's Creed IV: Ambergris Key
Note: this is an underwater location which requires a Diving Bell to access
 Also see
Assassin's Creed IV: Corozal Treasure Map (leads here)
Be sure that you have the Treasure map from Corozal before you make this dive to avoid repeat trips. While the cave is not the most complex, there are lots of jelly fish and urchins. Both of these things you want to avoid to keep Kenway healthy.
Swim through the field of jellyfish to get at the entrance of the underwater cavern. Just go forward through it until you reach the Air Pocket. Pause there to refill your lungs. Dive down, grab the nearby chest, and press on. Follow the path as it winds forward. This will lead you right to the ancient ruins. Be sure to head to the South and investigate the sand bar there to find the hidden chest you need the map for. From there head to the base of the temple and collect the Animus Fragment sitting there. After that move over to the Southwest and line up with the base of the pillar. Wait for the patrolling pirate. Lob the dagger at him to take him out while the second one is away (Rope darts work as well).
Head inside the main body of the temple to collect the rest of the chests. The animus fragment is somewhat hidden. Climb the wall in front of the door you entered from. This will get you up to it. The treasure map can be found in the Northwest. When you are ready to leave then climb on up the ramp in the Southwestern portion of the area.