Hi Richard,
Take a look at this campsite at La Tranche Sur Mer. There is a car park between the campsite and the beach but there is also a 'marine centre' next to the car park where you could leave your boat. The marine centre rents boats and boards, provides lessons and there is also a man made lake next to it. The campsite is very good with all of the facilities of a 4* site (swimming pool, kids playground, etc.). The campsite is good for tents, campervans and caravans and it is also possible to rent chalets. There is a card controlled pedestrian gate which leads out on to the car park and beach. The town centre is a few minutes walk away, along the pedestrian promenade:
http://www.camping-baiedaunis.com/
You could check it out on Google Earth.
The sailing club in La Rochelle is near to the town centre and main marina and there is no beach nearby. The nearest campsite is on the opposite side of one of the marinas, probably a fifteen minute walk away (or short pedestrian ferry ride):
http://www.larochelle-tourisme.com/ou-d ... -le-soleil
It is a fairly basic municipal campsite but it is very convenient for La Rochelle town (ten minute walk around the marina), which is a fantastic place for bars, restaurants, shops, history, etc., etc.
There are lots of campsites in north and west Brittany with beach access and also on the islands (Ile de Re, Ile d'Oleron, etc.).
I have only mentioned the two sites, above, as I stayed on both of them last September.
You could also look at this site:
http://www.eurocampings.co.uk/en/europe/
Ian (Yorkshire Dales S.C.)