Nice tip from Jeff Benjamin at iDownLoadBlog:

How to send a text hands-free with Siri on Apple Watch:

Step 1: Invoke Siri using Hey Siri
Step 2: Say something like “Send a message to Sebastien Page, hey man, how’s it going?”
Step 3: When the New Message interface pops up with the Cancel and Send buttons, say “Hey Siri, send.”
Step 4: The message will be sent without having to lift a finger

Jump to the article for more examples and a video showing this in action. Nice tip.