This page will have a guide to the treasure map you find in Salt Lagoon in Assassin's Creed IV Black Flag
Treasure Map Location
The treasure map itself can be found in a Cadaver on the Northeast side of the island, at coordinates (754,632), on the beach under a palm tree:
Treasure Map Tool
This is the treasure map you find:
It has a picture of a windmill, and coordinates indicate (442,118).
Note that the treasure chest itself is NOT located in Salt Lagoon (where you got the treasure map from), but rather in New Bone:
Dig Site Location
Dig location is around coordinates (442,115), at the east edge of New Bone:
To find the area, locate the windmill from the map:
Go east past the windmill, towards the shore. Near the water there are some palm trees growing, dig place is there.
See screenshots below for help finding the buried chest:
You get 4,000 Spanish Reales