The top photograph shows what you see with the naked eye. The bottom photograph shows what is seen through the polarising filter that is my sun glasses. Reflected light is plane polarised if the angle of incidence is above the Brewster angle. If the filter is aligned at 90 degrees to the polarisation plane, the annoying reflected light from the surface of the water is removed and you can clearly see the submerged causeway that is shown on the map. Speculation is that it may have been a glacial deposit then adapted for fishing purposes.
https://www.walkhighlands.co.uk/Forum/viewtopic.php?f=1&t=12876
