The watch really does notifications best. Those are a tiny amount of actual content with nice graphical wrappers and animations.
My wife and I often communicate in single letters and emoji. "U" "K" "👐"
I predict the Watch 2 will have a very custom Cell Phone chip to do notifications. Super low power, and optimized for receiving small amounts of data -- sent via either Apple's push notification service or text message and that's it. According to ancient FAQs, old school Pagers had very long battery life, measured in weeks or months on a AAA battery.
Hopefully Apple has the clout to either have it included in your cell phone plan, or baked into the cost of the watch so there's no monthly fee.
* Highly compressed voice for Siri is the biggest possible thing it might send. Looking a few minutes at the compression Siri uses, 10 seconds of audio could be about 5K to send, so very small too. Definitely something Apple could do.