Sonys W Series Walkman is a barebones MP3 player with one key selling point: its waterproof. But that can be a difficult thing to highlight on store shelves — at least when youre limited to traditional retail packaging. For its customers in New Zealand, Sony decided to try something different. With the help of ad firm DraftFCB, the company came up with a brilliant plan to help set its product apart from alternatives like Apples iPod shuffle. …