I found an apple watch and no-one is claiming it - if i could break the passcode I could find the owner

You can't "break" the passcode and Apple won't help you do so either. It's great (HONESTLY) you'd like to get the watch back to its rightful owner, but in these situations, you should turn it over to the authorities or lost and found if you found it somewhere where they have one and hopefully, whoever lost the watch has filed a police report.

Thank you again for wanting to do the right thing.