You can’t legally take possession of that boat or register it In California.The property owner cannot just give it away. The property owner may be able to take possession but there is a legal process to follow. Rather than consult an attorney and have to pay legal fees , call the local county sheriffs office. They are responsible for abandoned/derelict boats on waterways and can assist you in your situation for one abandoned on someone else’s property.
Before you get into this make sure the boat is salvageable. Most abandoned or derelict boats are real pieces of garbage. They cannot be raised from the dead.