@NathanG - I am more familiar with the literal usage of the term. For example, I think of empty soda bottles and things like that out on the waves of the ocean.
I suppose these items would qualify, although I think that the article is referring to items of greater importance.
I like watching television shows where explorers go deep sea diving to look for the areas where they think a ship has been wrecked. Sometimes they stumble upon a treasure; the ship itself, or parts of its oars or masts or other parts of jetsam and flotsam.
This is the stuff of treasure hunting, and it’s always thrilling to watch. If I understand the article correctly, I guess these discoveries would belong to the explorers, unless the government made a direct claim on the wreckage.