Unless I got the coordinates wrong, it appears to be part of a protected wetland area called Marais de Balençon. I can't find much more information aside from hiking trails and this very short description on the merlimont website.
I might be off by a bit though, navigating maps on a phone is a pain for me.
I found an explanation on a French blog. These shapes are called "Platières à bécassines". Bécassine is the French for the wild bird "snipe". The idea is to cut the grass/vegetation on some patches of the swamp in June (when it's dry), so that when water comes back in the autumn it creates small ponds that snipes like. It's for bird/duck hunting purposes basically.
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?
They may have started as squares! Those are peat bogs indeed, you can even see how they alternate and let it form again so they can get the good stuff in the future…
They are really strange. On google earth i was able to see some older picture of the same place. I dont think they are water ponds. In the earlier pictures its a bit more clear that they are earth patches
They dont seem to hold water really or even be connected to water at all.
If you notice, the first picture in the post comment, shows the same location slightly zoomed out. There you may notice some larger areas that seem like ponds. But the smaller patches seem to be just cleared earth patches