Hi,
the south west coast is really nice for surfing but the wind is not really consistant for windsurfing... But it can happen, some epic wave sessions... Otherwise the windsurf coastline starts with Ile de Ré and going further north (up to vendée, bretagne,...)...
L'ile de Ré is quite nice, Antoine Albeau F192 is the most famous local... :)
Have fun in france,
windsurfnews,
windsurfnews.blogspot.com