The big surprise when visiting the region of Aveyron (France) is to visit the village of Saint-Rome-de-Tarn. From the north side of the river Tarn (at Saint-Rome-de-Tarn) you have a good view on the gorgeous Cascade des Baumes.
Here the river Lévéjac drops dwon (in a single fall) into the river Tarn. Nothing special but the scenery is very special and gorgeous to see.
It looks to me a perfect spot to swim under a waterfall. It should be possible to get to the river and swim to the waterfall.
I have to guess, bu tit looks like a single drop waterfall of 25 meters, a must to visit when I am near.