Does anyone know if you need to have tethering data on your iOS device for the Apple Watch to function? I had this issue with Google Glass because I didn't a data plan with tethering, I couldn't access some features.
The apple watch is using an adhoc wifi and Bluetooth connection, similar to the principle of airdrop (device to device, no internet connection needed) to connect to the iphone. Your iphone will use data yes, but the apple watch won't use data. Let's say you get a photo in an email attachment on your phone, your phone downloads it, then sends it to the apple watch via the wifi direct, not using any more cell data.
Hope that helps.