There is an activities desk @ GCBC (on the beach that rents/loans equipment like sailboats or jetskis or whatever) and also a little kiosk in town, on the inland side of the street, situated in a parking lot possibly near Pressoir(my memory might be wrong on the exact location). I bet either one of them could hook you up w/diving.
As you're walking from Grand Case Beach Club into town, there is a dive shop on the left side of the street just before you get to the 'main' section of the street. It's just before you get to Le Ti Coin Creole. I know it's there, but other than that, I know nothing about it.
As far as restaurants, Le Ti Coin Creole is ok, but we were a little disappointed in the meal we had last week. He was out of the fish (this was early Friday night), and the chicken was a little dry and overcooked. The curry goat was good. L'Auberge Gourmande is perhaps my favorite in Grand Case - we ate there twice. Le Cottage is also really good. For good, cheaper food, the Lolos in the center of town offer more food than you need for a good price (for example, the 'Plate of Food' at Talk of the Town was 8 euros for a good portion of ribs and came with three hearty side dishes, I think the grilled fish was similarly priced).