Once we decided to wander off to nearby Trebes for lunch on the Canal du Midi somewhere which we did and on return to our place just saw a sign for Carcassonne Plage and curiosity got the better of us and there it was - a beautiful lake!
On one side good for a swim on the other side some fun ang game park on the lake with boat hire it seemed etc., three beaches and all guarded by a bunch of very vigilent lifeguards. We walked round the whole lake, it was very peaceful, only a few joggers and dog walkers about. It will take about an hour. Would recommend take picnic, above the lake in the trees are picnic benches and there is a lakeside cafe where cold drinks, beer etc., together with hot dogs, sandwiches, etc are all available.
Nice and safe place and plenty of shaded areas amongst the trees!