The notion that abandoned derelicts in international waters can be perpetually claimed by the original owners is silly. However, there are a couple of treaties in effect that protect sunken U.S. warships from salvage because they are considered graves and therefore the property of the United States, or whichever country it belonged to. This includes Spain.
This is more complicated than just being in international waters. My advice to the Odyssey guys is to start looking for other jobs, now every country in the world with a claim is going to hire a lawyer and take everything they recover.