There are six starfish in the beach area, all of which have to be thrown into the sea to unlock the achievement. To throw them into the sea, go to the fence next to the tent, which is the one place you can easily toss them into the ocean.
The locations of the six starfish are
1 and 2: Next to each other on the right hand wall, right after you leave the starting cave, before you reach the metal box which gets raised away from you.
3. On the right hand wall when you pass through the shipwreck, after the box and before the lit campfire.
4. Outside the tent.
5. Inside the tent, toward the back.
6. Inside the cave on the shelf just opposite the top of the stairs.