That is really theoretical and enforcement depends of individual cities. In fact a lot a naturist beaches are not separated from the normal ones by any physical limitation and you can go from one side to the other without noticing it.

I know it for having found myself surprised to be on the fun part of the beach countless times.

That's a picture of a French minister crossing path with a local enjoying himself : http://www.directmatin.fr/france/2016-08-11/emmanuel-macron-...

Oh, and topless for women is ok everywhere.