Not the Cote d'Azur, but try Camping Neptune at Grau d'Agde - there's canalside mooring, and the canal leads down to the sea. It's a lovely friendly site, too - and reasonably priced for a site with moorings. http://www.campingleneptune.com/english/index.html We've stayed there, and would highly recommend it. Lovely walk into Grau d'Agde and into Agde town (and driving distance to Cap d'Agde). Can't see a reference to boat mooring on this website, but the Camping Cheque website (below) refers to 'no extra charge for boat anchoring'!!!
If you're going between either of these dates 01/04 - 06/07 or
17/08 - 30/09 then you could use Camping Cheques, meaning a pitch would cost only