Looking along that coastline, there are similar areas at other river mouths, including the Somme. I found this blog talking about manmade ponds for observation or hunting of waterfowl. Wonder if that’s what it is?

http://www.gourmetfly.com/Somme.htm

Fleetwood Mac, Man of the World. “But I just wish that I'd never been born”. That one line alone does it for me.