Be careful if you're thinking about trying to rub the scratch out.
I've read elsewhere that some scratches might actually be micro scratches in the oleophobic (fingerprint / oil resistant) coating, rather than scratches in the actual Ion-X glass.
Rubbing with anything that is abrasive will almost certainly remove or damage this coating.
Please also note that the Apple Watch manual explicitly states that the coating will wear off over time.
"The front of Apple Watch is made of Ion-X glass (strengthened glass) or sapphire crystal, each with a fingerprint-resistant oleophobic (oil repellent) coating. This coating wears over time with normal usage. Cleaning products and abrasive materials will further diminish the coating, and may scratch the glass or the sapphire crystal."
From Apple Watch manual > Safety, Handling and Support > Important handling information
https://help.apple.com/watch/#/apd707b42a5e