Half Pendant of Aquatic Agility

This item plays a large part in the druid quest, part of the Aquatic Form quest chain.

When combined with the Half Pendant of Aquatic Endurance, this creates the Pendant of the Sea Lion.

Source
This item is found in different places for Night Elf druid than it is for Tauren druids. Clues to its location are given by the druids in Nighthaven, Moonglade.

Alliance: Darkshore
Follow the road north of Auberdine until you reach Cliffspring River bridge. Jump in the water and just swim straight out from shore. You are looking for a pair of rocks that, from the surface, look roughly like a crab's claw. They extend down into the water in a 'V' shape. Directly between them is a Strange Lockbox that contains the pendant half.

Also in the area are level 17 Threshers. You may or may not be able to wander between them without aggroing them.

Horde: The Barrens
Due west of Orgrimmar in The Barrens lies the Sludge Fen. Almost directly in the middle of it is a Strange Lockbox that contains the pendant half. There is a bubbly fissure nearby.

There is often a Sludge Beast nearby. This is a rare Ooze spawn.