I recently got this from a Southsea Cutthroat. The chests are too far and few for the still low drop chance they have, and mobs in instances have the same chance of dropping a world drop as mobs outside of instances. Therefore you're better off farming Southsea Pirates in Northern Barrens to get kills faster, avoid rolling for it, getting Steamwheedle Reputation and a chance for the Deckhand Shirt. Do it while it's not cross realmed.