Just before the floods in France (yesterday nothing was diveable, Landenouse could, but only 3 m viz) we dove the Cabouy (Reasseau de l'ouysse), this is a really big cave, it is divable from aprox may, but then only 3-5m viz. After end of september it shows its best viz, till the rain comes again. It is a real autumn cave to dive.
So in october it is most times really beautifull. We had extremely good viz, over 15m.