At the time, the projectiles dumped overboard from the CSS Richmond were legal to recover because they were not on or "in association with" (meaning, the debris field from) a shipwreck. That essential point tends to remain true today. Shipwrecks are protected. Stuff thrown into the river, no connection to a shipwreck, is not legally protected. To my knowledge, only one state has tried to claim everything that's underwater, namely, Alabama during the 1990s, which our esteemed fellow forum member Steve Phillips fought hard to get repealed, and succeeded. But I may not be up-to-date on recent deveopments in "Maritime/Riverene Archeology Law.