You need to be mounted on a yak (not grand expedition yak), so Blonde Riding Yak, or Grey Riding Yak, and then use the toy. You will then get a 15 min buff "your yak is wet" or some such. Any mount you summon will be a yak for the next 15 minutes with the buff, and still be able to fly (if you summon a flying mount). Azure water strider also still waterwalks, even though it looks like a yak, too. Hope this helps.