In Alabama it's easier to register an abandoned boat if it doesn't have any numbers.
If it has a Hull ID, contact DMV/Marine Police and see if it's been registered. If not, get a bill of sale from property owners, and go get it registered.
If it does not have a hull ID, contact Marine police, Issue new Hull ID, Get Bill of sale from property owners, and go get it registered.
If it has numbers, or Hull ID came back to an owner you must have bill of sale from last registered owner. Or even worse a title in some states.. If it's been 10-15years since last registration, usually tag office will let you slide... Still need a Bill of sale so they know how much to tax you.
It is a felony to forge signatures on official documents, so it wouldn't be worth creating a song and dance. You could ask permission and usually previous owner will submit to prevent a hassle. If there aren't any numbers on that boat, of if there is and they don't come back to previous property owner contacting him would be a waste of time. Pre-72'? Boats were not required by law to have HIN so if it's older don't be surprised not to find one.
Boats only require registration if they are powered by a motor in public water. Bill of sale for boats in my state require HIN,Reg#,Owners Name and sometimes address where it was last registered, amount and date it was sold, also two witnesses.
Think I've gotten at least 5 or 6 boats through the process that were lost in translation or never registered. It's not a difficult process surprisingly in this state though I wound not spend a dime on it until legally its your property.