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 ([49, 11]). 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.