This area has many open water swimming areas from lakes to hidden away mountain rock pools.
The local town of Quillan (10 minutes drive) opened a brand new swimming lake in 2016 and has its own blow up water park during the season. There is a ‘beach’ area with lifeguard throughout the summer months, small café / restaurant and toilet/changing facilities.
Puivert also has a similar sized lake with amenities (30 minute drive).
Belcaire, slightly further up the mountain towards Camurac (40 minutes) is also a popular spot for swimmers.
Lac Montbel is one of the largest man-made lakes in Europe and is a fantastic place for swimming and other watersports.
There is also a large lake at Carcassonne, Lac de la Cavayere, with large swimming area and watersports available.
Follow the link for further information – https://www.pyreneesaudoises.com/nature/lacs-des-pyr%C3%A9n%C3%A9es-audoises/