In the center of the map is an underwater shipwreck.
There is an underwater shipwreck with 3 Chests in the center of the map. There will always be 3-5 Sharks in the area. The most dangerous part is swimming from the boat to the sunken ship (and back), but it's a short distance. Once inside the ship, sharks can not attack.
There are 9 empty chests in the upper cabin, and 3 lootable chests beneath the floor of the upper cabin.