Okay, let's get this straight. $500 million worth of treasure from a spanish galleon ends up on the bottom of the Atlantic and in international waters. Some folks from Florida find said treasure. Spain sues.
Spain has forgotten the very simple rule involved here: Finders Keepers.
Now, granted, the U.S. government is not much better. The U.S. Navy had a F3A-1 crash on someone's back 40 and left it there for over five decades. Someone then picks up the Navy's litter and starts restoring the plane. The Navy sued, until they were backed down through the intervention of Congressman Walter B. Jones, who quite rightly could tell that this was best described by the title of Penn Jilette's reality show on Showtime.
The rule is simple, you drop something in the ocean, particularly in international waters, pick the damn thing up. Otherwise, someone else will pick up your litter, and you will have a hard time getting it back.