stolen or loss apple watch
my apple watch is missing lost or stolen how do i report it stolen
Apple Watch Series 5, watchOS 9
read
If your Apple Watch is lost or stolen - Apple Support
you report it stolen with your local law enforcements and your insurance company
and if it's a cellular apple watch with your carrier having them block the IMEI number
you don't report it stolen with apple as like car manufactorers and the likes don't act the role of investigator of products they sell to customers being lost or stolen.