The visible watermarks primarily aim to deter unauthorized use rather than to prove copyright claims or help people locate a copyright holder. If you want to prove copyright claims, there is a centralized registration system that has existed for over a century

https://www.copyright.gov/registration/index.html

Locating copyright holders is a much thornier problem

https://en.wikipedia.org/wiki/Orphan_works

but it's not obvious that a decentralized database is more effective somehow than a centralized database.