The data on an Apple Watch should be automatically backed up on the same iPhone the user of the Apple Watch has. When the watch is first unpaired from the smartphone, it will initially back up its data to the iPhone. A back-up of the iPhone data to iCloud will include the Apple Watch data.
The Apple Watch back-up will include app specific data (including data from Maps); general system settings; and health and fitness data.
The Apple Watch synch will not include:
1. credit card information used for Apple Pay
2. text messages
3. the passcode for the device.