Originally Posted by Unregistered
I assume you're referring to the one that is on the tall rock with no way to get up it. From the beach, go up and around (close to the treasure that is on the side of the cliff on a small shelf). You will see the treasure on the rock across the water. Use your 4 wheeler to jump over the beach and water to the top of the tall rock to get the treasure!
Just realized you said in the center of the island. This one (or acually two) is located on the bottom right of the map near the jump-rock and bungee.