Instructions
Acquire Starfish Offering
Enter the Museum of Olympus near Herc's Hero Hut and walk right until you see the statues of the gods.
Locate Poseidon's statue. It will have a starfish covering the face.
Jump up to the statue. Click the starfish to remove it. A Starfish card will transfer to your "Items" bag.
Enter Poseidon's Realm
Leave the museum and go right past the temples until you reach the hillside you couldn't access before. Walk up the hillside to the cliff and then leap off the cliff into Hades' realm.
Go right until you reach a platform rising into the air on a column of mud like an elevator. When the platform returns to the ground, jump onto it. Wait for it to carry you to the top and then jump right to enter the outer area of Poseidon's temple.
Enter the temple and walk right until you reach the entrance to his realm. Open your "Items" bag and click "Use" on the Starfish card. It will appear on the column next to the entrance and the entrance will open.
Enter Poseidon's realm, walk down the beach and then into the ocean. Swim out to the statue to the square-like area in front of it that is a different color then the surrounding water.
Get the Pearl
Hold your cursor above the right side of the area you're in to see a "Dive" request. Click once when you see the request to swim underwater.
Swim through the maze of underwater rock walls and ledges to the clam. Go down, swim right and up to the ledge above on the right. Swim right along the ledge, go down, swim right and then down, swim left slightly and continue down. Swim left again and down, swim left past the shipwreck and then up to the clam resting on a ledge to the right.
Wait for the clam to open and then swim into the opening to get the Giant Pearl. The "Sacred Item - Giant Pearl" card will transfer to your "Items" bag. If the clam whacks you as it closes, wait for it to open and then try again.