See the manual, page 18:
Use Apple Watch without its paired iPhone
Although you need an iPhone to do most things with Apple Watch, you can still do several things with Apple Watch without having an iPhone in range.
- Play music from a synced playlist on Apple Watch
- Use the watch, alarms, timers, and the stopwatch
- Keep track of your activity (stand, move, exercise) with the Activity app
- Track workouts
- Display photos from synced photo albums
- Use Apple Pay to make purchases. See Make purchases with Apple Pay on page 58.
Apple Watch uses Bluetooth® wireless technology to connect to its paired iPhone and uses the iPhone for many wireless functions. Apple Watch can’t configure new Wi-Fi networks on its own, but it can connect to Wi-Fi networks you’ve set up on the paired iPhone.
If your Apple Watch and iPhone are on the same network but aren’t connected by Bluetooth, you can also do the following on Apple Watch without iPhone:
- Send and receive messages using iMessage
- Send and receive Digital Touch messages
- Use Siri
https://manuals.info.apple.com/MANUALS/1000/MA1708/en_US/apple_watch_user_guide. pdf