There are some large shells (4 dark shells and 1 yellow one) across 3 different screens that you need to click on to open them up for this achievement.
The first two shells are in the first scene when you are underwater, on the right of the submarine. Click them both so they open.
The next shell is one level up from this previous scene, on the far left. Click it so it opens.
The last two are one more level up, on the far left. Click both of the large shells so they open.
Achievement unlocked! :)