Mostly the diving is tourist reef and general stuff. That reflects the folk in the area - tourists mostly. Vis can be patchy and gets better as you go west. The area round Argeles sur Mer offers some pretty good diving but its a fair drive, although easy enough down the motorway.
The lake is total shit - don't go there. You will regret it. I don't think there's any diving but there are lots of camper vans and dogshit (its France after all ) There were millions of mozzies when we last went (I mean like millions). However, from Clermont l'Herault the town near Lac Salagou you can drive north up the A75 to Lodeve and then into the Cevennes. Head to Le Vigan, from there to Ganges and then back to Clermont again. The scenery is some of the best and most spectacular in the world. Look for Cirque de Navacelles too, there's a viewpoint with a lovely resturant.
There's a teleski (water) at aquamagic in Port Leucate if you like that kind of thing - great to occupy the kids for half a day. Diving there too.
Don't forget you need a medical of less than a year to dive in France.
If you want more info on the area PM me - we have a flat in Montpellier and love the area. The cathedral at Narbonne is breathtaking, but Beziers is not my kind of place TBH.