The ships may be well preserved, but the cargoes are another story. Crews of wrecked ships would salvage as much as they could. Other cargo items were looted, many by divers over the last century. It was not illegal then to bring back items that could be sold or used as souvenirs. These were in the days before the modern divers credo "Take only pictures, leave only bubbles."
One funny story, though is the toilet and anchor from the barge Herman H. Hettler, which sank in 1926. A local diver snatched these two items in the 1960s and proudly displayed them on his front yard until he retired to Texas 30 years later.
That's when Munising locals retrieved them (plucking flowers from the commode that the wife had planted) and re-sank them in the ship. This explains why the toilet sits perfectly upright on the wreck.
Sounds Like An Interesting Place to Visit. --RoadDog