Where is the Hydra on Mythology island?

The Hydra is in a large air-filled cave at the bottom of Poseidon's underwater maze. Dive in from the beach, swim along the top, right side, and bottom to reach the cave. The bubbles are oxygen refills, so it is hard to run out. The giant pearl is just outside the cave.

In Detail

Enter the realm of Poseidon through the temple (far right of the island). Use the starfish from the Museum of Olympus to enter the door. On the beach, Aphrodite will give you a test and reward you with a valuable mirror if you pass. (The answers are Ares, Hermes, Aphrodite, Artemis and Hestia.)

The Hydra cave is underwater off the beach. Dive down and swim along the top, right, and bottom of the maze to reach the cave (above the cave is the Giant Pearl).

You can defeat the hydra by jumping up when it strikes at you, and landing on the attacking head. Finish all five the same way before any can wake up. Swim below the Hydra and you can get the scale from it.