I understand that, but having spent over 20 years in the Navy, on 4 differant ships, I can tell you sea water isn't kind to metal objects.
Its a plausble theory. Notice I said theory
The problem I have with the “ancient mine” theory is that the explosion appears to have occurred at the stern of the ship. If it were a floating mine, I would expect it to have occurred at the bow.
Modern mines, however, are different animals.