In a curious coincidence, there was a question about this part of France just a few hours ago.
You are looking at the Noirmoutier area of France, which in the past was one of the world's premier regions for the production of Fleur de Sel:
a salt that forms as a thin, delicate crust on the surface of seawater as it evaporates.
It is produced thus:
One method of gathering sea salt is to draw seawater into marsh basins or salt pans and allow the water to evaporate, leaving behind the salt that was dissolved in it. As the water evaporates, most of the salt precipitates out on the bottom of the marsh or pan (and is later collected as ordinary sea salt), but some salt crystals float on the surface of the water, forming a delicate crust of intricate pyramidal crystals. This is fleur de sel.
The map you include in your question shows salt pans. For more on this intriguing topic, I can heartily recommend 'Salt: A World History' by Mark Kurlansky, available from the usual booksellers.
Tourism forms a major part of the economy of Noirmoutier these days, and you will find many places willing to sell you some genuine Noirmoutier fleur de sel.
However, as pointed out by peufeu, the particular area you show may in fact not contain salt pans (although there are a lot in the area) - what is shown here may be oyster ponds for in-shore shellfish farming, another activity common in the area.
This tourism info page about the area includes this image of a fishery:
and the text
In the sheltered waters of the bay, in-shore fishing and oyster
farming are important industries in the small communities of Bouin and
Beauvoir sur Mer.
Also, compare and contrast this picture of salt-farming:
and this picture of oyster farming:
both from another tourism site for the area. The more irregular water features you're asking about seem more like the latter.
Still read the salt book, though :)