There are human-made structures and other signs of civilization just under the sea surface in many parts of the world, but not from as recent a time as Hog Island. This one you link to is fairly easy to explain, due to the fact that it was little more than a large sand bar that had built up enough to erect a few buildings on before eventually being washed away. Of course, the 'fact-checkers' would deny it and we'd be expected to just believe them.