I found an Apple Watch and is locked, can I deactivate it

Absolutely positively NOT. And you shouldn't even have any right to use someone's lost Apple Watch.

What you should do is the right thing and turn it into the authorities where you live and hope the owner who lost their watch, filed a police report and is reunited with their watch. It's totally, utterly, 100% useless to you. Apple will NOT help you use the watch either.

Do the RIGHT THING.

No.

Doesn't matter where it was found. You don't have the right to try to use it. Since it was found from an airline trip, contact the Airline and arrange to return the phone to them. Surely, the owner contacted the airline to ask about their watch.