Trial of the Sea Lion
The Pendant of the Sea Lion is formed from two parts: the Half Pendant of Aquatic Agility and the Half Pendant of Aquatic Endurance. Before you can obtain these two items, you must accept the "Trial of the Sea Lion" quest from Tajarri, who is located at the Shrine of Remulos in Moonglade.
Half Pendant of Aquatic Endurance
For Alliance, head to Westfall and swim into the ocean that is west of Gold Coast Quarry. Keep your eyes on the ocean floor as you swim and soon you will arrive at a large anchor. Swim down towards it you should see a Strange Lockbox nearby, which contains the first half of the pendant.
For Horde, head to Silverpine Forrest to the beach just west of The Sepulcher. Locate the wrecked ship on the beach and swim due west, keeping your eyes on the ocean floor. Swim into the large fissure that opens up and loot the Strange Lockbox at the bottom, which contains the first half of the pendant.
Half Pendant of Aquatic Agility
For Alliance, travel to Darkshore and follow the road north out of Auberdine until you reach the Cliffspring River bridge. Swim out away from the bridge and you will eventually encounter two large rocks underneath the water's surface. Submerge between them to locate a Strange Lockbox, which contains the second pendant half.
For Horde, travel west from Orgrimmar into the Sludge Fen located in The Barrens. In the middle of the Sludge Fen you will find a Strange Lockbox, which contains the second pendant half.
Combining
Bring both halves of the pendant to Tajarri at the Shrine of Remulos in Moonglade, and she will combine them to form the Pendant of the Sea Lion for you. Although this item will no longer grant you Swimming Form, it can be turned in for experience, money, and counts towards the "Loremaster" achievement.