The WhatsApp messenger app available in my regional AppStore does not include an Apple Watch App. Now, maybe the one in yours does, this may vary, but I doubt it. If one is available it will be mentioned in WhatsApp card in your phone´s AppStore. You can also find out through the Watch app on your phone through My Watch tab and scroll all the way to the bottom. Apps available for your watch will be at the bottom with an Install button or an Installed tag.
Also, please use Apple Watch to designate your watch, Apple never made an iWatch.