So this achievement unlocks when you destroy all 14 of the hidden skulls around the island. There are 4 outside the island that you have to snipe, so bring a sniper rifle. The remaining 10 are hidden on the island itself. I'll list them best I can. I recommend grabbing one the elite soldier's uniform (guys in red that roam around in the upper half of the map) to make this as painless as possible.Notes:
When I give NESW directions, follow your in-game compass to avoid confusion, as North faces right for some stupid reason.
-Make sure at least 2 targets are dead so the storm is active, as that’s the only way the ghost ship will spawn after you get the skulls, which triggers the achievement. (Credit to GameXpert5000)Outside the island:
#1: From the starting point, turn around. To your left will be a plane with an open door. One in there.
#2: Looking to the southern island, there is one perched on a foldout chair.
#3: Upper right side of the map, it's in the window of the sunken boat
#4: Going to the villa, you should see a yacht out on the SW waters, there's a skull on the front tip of the boat.On the island:
#5: In 47's room that you're given, on the bathroom sink counter.
#6: Underneath the gym, there are two guys playing video games, it's right under the tv in the shelf.
#7: Middle-leftish on the map, there's a very tiny bar, it's in the shelves.
#8: The spa/massage area. Entering the building, it's on the opposite side of the wall as the reception area, in a long shelf by some towels.
#9: Upper right of the map with all the elite guards, it's in the little hut on the shelves over the bar.The rest can be found in the large villa to the north. BRING YOUR SNIPER RIFLE:
#10: Ground floor, from the entrance go left into the room with the fish tank. It's in the fish tank (This guy took me 5 hours. You're welcome!
#11: Going upstairs into Tyson's bedroom, look into his large closet on the ride side of the large shelf. You'll see the skull.
#12-13: Upstairs in the movie theater room. One is on the shelf above the couch, the other is in the back corner of the room next to the Boba Fett helmet.
#14: Having your sniper rifle with you, look out the large front window of the mansion. It's on the very top of the big tower in the center of the island. From this vantage point you should be able to shoot it with slowing time (press RT slowly before taking your shot)
Edit: Video guide for those who prefer a visual, courtesy of Maka91.