How good and clean are the beaches?
It will depend where in France. There is an international organisation that rates all beaches called Blue Flag. In France, it’s called Pavillon Blue, and on their web site (unfortunately only in French) you can see which beaches and which marinas have received the coveted “blue flag”.
The Pavillon Blue (blue flag) is given to those beaches that meet a very big series of criteria, not only ranging from clean water and beach, but infrastructure, lifeguards, signages, access, ecological, etc.
On the Pavillon Blue web site (in French), you can see a map with all the beaches and marinas. You can zoom into the map to the beach you are interested in.
Once you are at a beach, and you want to know if it has been approved, look for the logo, usually prominently displayed (mind you, this is not just for sea beaches, but also river beaches):