Undersea Cave

Once you rescue Poseidon, head south of Leneira to visit him in his shrine. By stepping into the portal behind him, and dropping down the pit, you'll end up in this small cave. Inside the cave is the Coral Armor, which is nice, and Poseidon's ship, which allows you to dive underwater to avoid enemies and reach areas you couldn't with Homer's ship. Speaking of which, you'll need to use the ship's diving abilities to reach either of this cave's two exits.