This page has details on the treasure chest buried at coordinates (749,625)
Treasure Map Location
The treasure map for this treasure was found in Abaco Island at the southwest part of the island:
Treasure Map Screenshot
Screenshot of the treasure map:
Treasure Chest Location
The treasure chest is found in an island called Salt Lagoon.
Where to dig
Go to the northwestern part of the island and find the pile of branches on the ground:
Dig locations is there. Run around until you see the dig option.
Loot from this chest is 3000 Spanish Reales and plans for a serpent figurehead: