Fish up a Royal Monkfish from the moat in the Valley of Heroes district of Stormwind. Once you catch a monkfish, fillet it and check if it swallowed the locket.
Arnold and I used to relax down at the moat in the Valley of Heroes. One day, I lost my locket in the lake and I have never been able to find it. Rumor has it that huge royal monkfish live in that moat and that they eat anything! I know it's a long shot, but do you think you could catch a few monkfish and look for my locket?
Please let me know right away if you find it, it's very precious to me.