Hi
Some of the built-in apps on Apple Watch - including Mail - can be removed from your watch. However, this can only be done by removing the associated app on your iPhone. It is not possible to delete built-in apps only from your Apple Watch.
To remove a built-in app from your iPhone - and therefore also from your Apple Watch:
- On your iPhone: touch and gently hold your finger on an app icon until all of icons start to jiggle.
- For apps that can be deleted, an "X" will be shown next to the icon. Tap on the "X" to delete an app.
- Press the Home button when done.
If you would prefer to keep the Mail app on your iPhone, but not to receive Mail notifications on your Apple Watch:
On your iPhone, in the Watch app, go to: My Watch (tab) > Mail:
- Choose Custom.
- Turn off Show Alerts.
To remove third-party (ie non-Apple) apps from your Apple Watch:
On your iPhone, in the Watch app, go to: My Watch (tab) > scroll down to the list of apps > tap on an app's name > turn off Show App on Apple Watch.
You may also wish to turn off Automatic App Install:
On your iPhone, in the Watch app, go to: My Watch > General > turn off Automatic App Install.
More information:
Remove built-in Apple apps from the Home screen on your iOS device with iOS 10 - Apple Support
https://help.apple.com/watch/#/apd99e3c6a68