To start with, open Chapter Select and go to "Cove". It should be the near the bottom of the list.

Follow the water until you get to the left-most triangle, which I am standing right under in this screenshot:










I am facing the island, my back is to the ocean.

From this point, turn around to face the ocean, then follow the water to the RIGHT. Eventually you will come to find this under the water:











When you walk through this, a spherical rock comes out of the ground. Interact with this, and the achievement should pop.