This achievement can be located at these coordinates X: 619.7, Y: 559.4 on the main island. It'll be easier to get later on in the game when you've unlocked more of the island, but it can be obtained at any point.
Once you've reached the location, look for the bodies hanging from the tree. A little to the left side of them (back to the water) and buried into the sand you should look out for a head popping out of the sand. Head over to it, then press and hold on the head to unlock the achievement.