Hi Roscommon
We spent some time in Marenne 2 years ago. It's a lovely area. The zoo at La Palmyre is excellent and La Rochelle Aquarium wortha day out as Michaelm says. I have never seen fans for hire at the campsites and in August you would be lucky to get one anyway. It will be hot in August but you may get some thunderstorms or showers which will cool it down.
If you like wine, there is a very good wineshop on the road into Marennes from the McDonalds restaurant. The proprietor lived in Ireland and has excellent English. Wine €1.50 a litre. yum! We stayed out at Ronce le Bains, nice little seaside village near Marenne. There was a funfair there permanently and a few nice restaurants. Great forests along the coast behind the beaches, sea very rough, and lots of cycle trails. Near MArennes are the canals the fishermen use, lots of oyster/mussel shacks and restaurants.
You will enjoy the area.
Slim