It turns out that maritime law has all kinds of jargon for different kinds of wreck, a whole universe of nautical meanings for things I never thought to remember to check on. All different kinds of consequences for each of them.
So what I now know is that flotsam is what you find floating on the surface, the wreckage of the ship or what was inside. Jetsam is what you throw overboard in distress, all of the machinery and barrels of spare cows and bales of love letters that eventually either sink or wash ashore. Lagan is what you leave on the bottom of the ocean to be picked up later, when the storms have blown over or the ships have gotten bigger.
Obviously, for my purposes, it's the derelict that we're interested in, which could turn out to be all of the above--all of that which is on the bottom of the ocean but without hope of being reclaimed. Just another wreck. In the laws, it turns out that dereliction is at least initially all about intent--if the crew is abandoning something without planning to go back and find it later, it starts out as a wreck. In those cases it seems like the only redemption is in salvage, in finding what was abandoned and reclaiming it from the waters.
Maritime law gets poetic when it talks about salvage, maybe because salvage is less law and more suggestion. Still, in 1989 they amended the previous convention to include acts of attempted salvation with little chance of success, if what they're trying to save will otherwise be irreparably damaged. The conventions are all talking about compensation, of course, but I think that what is gained either way is still not quite the point.