Even though Siri's getting a massive update in iOS 12 later this year that makes it far more versatile, it's not quite the most powerful voice assistant around at the minute. Until now, it was the only one available on Apple Watch. If you've wanted to use Alexa on Apple Watch, though, third-party app Voice In A Can brings Amazon's tool to the device.
This is a standalone app, so you don't need your phone around for it to work, only a data or WiFi connection. Developer Damian Mehers recommends adding the app as a Watch complication for easier use. Once you've installed Voice In A Can and linked it to your Amazon account, Alexa can do Alexa things on your Watch.
There are some limitations: it can't play music, audiobooks or podcasts. Some daily flash briefings don't work either. You can, however, control smart home items you've linked to Alexa and take advantage of its other features. Annoyingly, your Watch will disconnect from Alexa when the screen goes black, so you might want to increase the timeout limits. Since you'll need to have the screen awake and to tap a button to activate Alexa, that shouldn't pose too much of a problem.
It's not unfeasible that Amazon will add official Apple Watch for Alexa down the line, with more integrated functionality, but at least this works for the time being. Just note that Voice In A Can ($1.99 in the App Store) could vanish if Apple or Amazon decide they don't like it.