Hi Anthony,
Was there two October ago, and there are a fair number of ops between Marseilles-Monaco, but I didn't think much of the admitedly limited diving I undertook.
In Hyeres, a bit NNE of Toulon, check out the local Destintion Plongee chain shop: Joël Rimbaud Avenue de l'Aéroport - 83400 Hyères Tel. 33 - 04 94 38 35 85 Fax 33 - 04 94 38 30 72 Minitel: 3615 SCUBAPLONGE. The diving of Porquorolles is okay.
Couple of shops in Cagnes sur Mer, a bit WSW of Nice, are Ancre d'Or 84 bd Plage, BP 64 06802 Cagnes Cédex 93 31 02 10 & Société Nautique Cagnes/Mer rue Capit Frégate Vial 93144800-93075993.
The above shops accepted my PADI cert, but some French ops want CMAS (World Underwater Federation).
Best of luck.
DocVikingo