To find a buried treasure, you must first obtain a treasure map. A
treasure map can easily be found on Mariguana Island, at the coordinates
878, 539. Collect the map, and you will be shown a picture and the
coordinates of a treasure. This particular treasure can be found on
Andreas Island, at the coordinates 575, 717. To unlock the locations of
other treasure maps, you can talk to the barkeepers of taverns, or just
explore the world and synchronize viewpoints.