I'm not an expert on the law of the sea, but it would seem that after NASA abandoned these engines in the ocean 50 years ago, they would belong now to whoever could salvage them.
"Salvage" has a very specific legal definition, it means rescuing property that has been abandoned.
NASA didn't abandon the engines, they could claim that it simply wasn't cost effective to retrieve them.
A related case is the treasure hunters who discovered $500M worth of gold and silver coins on an old Spanish ship in the middle of the Atlantic. The Spanish government sued for recovery, and won:
http://www.rawstory.com/rs/2012/02/17/judge-rules-treasure-h...
That case made it all the way to the US Supreme Court, who did not overrule an earlier court ruling that Spain had sovereignty and property rights over the treasure.
In the case with the engines, Bezos' initiative is purely non-profit, so I doubt he and NASA will come into conflict. I believe he has said that the engines will be given to the Smithsonian.
If, however, somebody were to retrieve the engines for financial gain, NASA would have a case against them, and it is much easier to asset ownership over something from 50 years ago than it is with a sunken vessel from 250+ years ago.