Scratch on watch face

Apple would consider a minor scratch to be cosmetic damage and as such would not be covered by the warranty or even an extended warranty (AppleCare) unless it could be proven to be manufacture error.

https://www.apple.com/legal/warranty/products/warranty-us.html

This Warranty does not apply: (a) to consumable parts, such as batteries or protective coatings that are designed to diminish over time, unless failure has occurred due to a defect in materials or workmanship; (b) to cosmetic damage, including but not limited to scratches, dents and broken plastic on ports unless failure has occurred due to a defect in materials or workmanship; ...

You will just have to live with the scratch. Watches are not "scratch proof". That is why they sell screen protectors for the watch. Personally, I have found that a wide piece of cellophane tape works just as well. It is cheap and easy to replace.