OK, here is one for you. I was the assistant project director for a shipwreck excavation in Matagoarda Bay off the coast of Texas. This is in saltwater mind you. The ship sank in 1685. That is over 300 years old. We found over 1 million artifacts. That is right 1 million.
This was also an area were very few divers ever went. Who is to say how long things will last underwater whether they be in fresh or salt water.
Archaeologist would love to go pick up everything and then have it all conserved and placed in a museum for everyone to see forever. Now, archaeologist don't have enough money to do this, state and federal governments don't have enough money to do this and the private sector doesn't have enough money to do this. Over time, more are being excavated or at least studied.
I know the argument, if left they will be destroyed. Well, if removed, are they any better off? Only the few who took them will ever know of their existence.
Just my 2 centsL