how to find the serial How to find the serial number or IMEI for your Apple Watch – Apple Support (UK)
other than that
the activation lock is not really on
the watch it's a theft protection measure on apples servers
it needs to be removed
About Activation Lock on your Apple Watch - Apple Support
if you don't know the appleID
Recover Your Apple ID - Apple
if that does not help you then you
need the assistance of apple
Additional Support - Activation Lock (apple.com) and or
Get Support (apple.com) , Genius Bar Reservation and Apple Support Options - Apple