government agencies do not relinquish their claims lightly. They don't consider their hardware to be "abandoned", merely "very hard to get to".
My ex-coworkers at the Museum of Flight (in Seattle, mentioned in this article) know the exact location of a WWII-era fighter aircraft that crashed in Lake Washington. They recovered and restored one aircraft from the crash site at least a decade ago, but the Navy will not let them touch the other one.