There are two local swimming pools, both outdoor and open during the high season at Axat and Quillan.
Alternatively, there are swimming lakes with ‘beach’ areas at Quillan, Puivert, Belcaire and Montbel Lakes. All have lifeguards in high season, but you are able to use the lakes for most of the year (weather permitting!)
See link for further information – https://www.pyreneesaudoises.com/nature/lacs-des-pyr%C3%A9n%C3%A9es-audoises/
There is also a large lake at Carcassonne, Lac de la Cavayere, with large swimming area and watersports available.