Ok so I got it to work in Beta 3. You have to go into settings/Siri/ and turn on announce notifications and pair Airpods with the watch. But its the same as on the iPhone, it toggles announcing things through the airpods.
EDIT: Well it may not need the airpods, I moved them to my phone and I still see the option in my control center so you probably just need the announce notifications setting turned on in the settings app of your watch