Le Marine, Noirmoutier
Several years ago, we ate lunch at a restaurant in the Ile de Noirmoutier named Le Tire-Bouchon. I had aile de raie aux salicornes - first time ever to eat the latter - and was astonished at the flavor of both the fish and the salicornes. Not sure if the place still exists, but the lunch was memorable. We then bought jars of salicornes in nearby stores. Don't know where else in France one can find these, but glad we did on that summer day.
I recommend it: good creative cooking, great produce, great wines. But do not fail to have one or two meals at La Table d'Elise, the bisto next door, operating from the same kitchen. The food is less "haute" but highly satisfactory. And if you want a plateau de fruits de mer, this is the place.
If you are not horrified by modest people's cooking, at the exit of L'Herbaudière going Southwest (towards the capital, Noirmoutier-en-l'ïle) is one of the few remainings ("Workers' restaurant"), Au Vieux Loup de Mer, serving simple but fresh and generous food at low prices. Do not miss the grilled sardines.
Off-topic perhaps but a serious caveat: by no means should you ever stay at Le Château du Pélavé. I mean it.
If you like mussels, you can have them anywhere, but the very best I had were at Tantine Berthe, at Le Grand Vieil.