The French Riviera has a Mediterranean climate, with sunny, hot, dry summers and mild winters. On the Cote D'Azur there are many good diving spots, but most of them tend to be rather deep. The best sites are in the Bay of St. Tropez, where you have relatively shallow reef dives, but deep wreck dives.
Corsica's inlets and beaches are numerous and the island possesses 30 diving centres dotted along its 1000km coastline. The rugged landscape on the dry side of the shoreline continues underwater giving shape to mountains and canyons. Corsica's underwater kingdom has been really well preserved due to the absence of an intensive commercial fishing industry and an almost total lack of polluters and pollutants. Corsica also has many diveable wrecks. Bonifacio, in the south, gives access to the Lavezzi Islands and its marine reserve.
In northern France, the main diving area is specifically around the D-Day landing beaches and coastline of Britanny. Arromanches is a popular spot for diving with hundreds of wrecks. Many dangerous wrecks have been cleared, but The D-Day Wrecks museum in Port en Bessin can give you a good idea of what is still out there.