Trajanus
Paranormal Adept
The time elapsed alone means that any evidence left of any form in such an environment will likely be long gone.
If you mean by natural decomposition or degrading I don't think we can assume that. Researchers have found many old shipwrecks, or remains of them. In fact so many are known from the Roman period that diminished numbers of shipwrecks after the second century have been cited as evidence for economic decline.
Not that I'm expecting them to find a saucer wreck.