Seriously, just look on Google Earth for a lake near to where you are intending to stay and if it has a plage you can swim there. There are hundreds of communal lakes with plages.
------------- If I had known how busy retirement would be, I'd have carried on working.
Admittedly it was many years ago but a village/town called Bougeuil near to Chinon had a very good lake with pedaloes and swimming. Don't know if still there. Like I say it was a long time ago I was last there so maybe change now.
------------- Linda
2017
May - 3 nights Yeatheridge.
May - 5 nights Mill Park, Ilfracombe
July - 6 nights not sure where yet
August - 2 weeks Croatia, Camp Lanterna.
2018
May - 3 night Yeatheridge campsite
May half term - 6 nights Cornwall
July - 1 week Cotswolds
August - 2 weeks Vendee.
Don't forget to leave a review of the French and other European campsites you have visited!