my Apple Watch is iCloud locked.

Having the serial number won't help you. If the watch is locked to another user, ONLY the user who locked the watch can unlock the watch. If that isn't you, no one can help you. Apple won't help you.

I'm going to guess you either purchased or found a used Apple Watch? If you purchased one, you MUST get the owner who locked the watch to remove the watch from their account. If you don't know them or they can't remove the watch from their account, the watch is utterly useless to you. Try to get your money back if you bought it. If you can't get your money back, recycle it responsibly, unless you want to wear a totally useless wrist ornament which can never function as a watch.

Read this too --> https://discussions.apple.com/docs/DOC-250001038

If you set the Activation Lock please see Apple's notes in About Activation Lock on your Apple Watch - Apple Support

If you have acquired an Apple Watch which has Activation lock set by a previous owner, only the owner who set the lock can release the watch from their Apple ID. If you can't contact them or they refuse to unlock, the watch is useless and can never be reactivated.

My watch is locked for my iCloud