Evidence has been mounting over the last few days and it looks like it's finally happening: Android 6.0 for Wear is starting to roll out. Googler Wayne Piekarski just announced on his Google+ feed that OTAs have begun and should continue over the next few weeks.
An official blog post by Google lists some of the new features we can expect in the new firmware, including: newly navigation gestures, audio support on speaker-equipped watches, and expanded support for messaging clients.
In Piekarski's post, he reminds developers that the API 23 SDK is already out, but that nobody should remove support for API 22 until the rollout is complete. It's not specifically stated, but this reminder does clearly confirm that all Android Wear devices will be receiving Marshmallow, otherwise the advice would have been to continue supporting API 22 for any device left behind. Those with a Gear Live, G Watch, or original Moto 360 won't have to feel left out of the fun.
From the sound of it, the first OTAs will start rolling out sometime today. Some models will receive updates before others, so don't worry if there's not an OTA floating around for your particular model. It's likely it wasn't on the list to go first. Like all Nexus OTAs, a small group for each model will be first to get updates, and then the numbers will ramp up quickly if no major issues are reported.
It's time to start checking for updates!