I lived in St. Maarten for some time. Hard to go wrong in grand case, but the best, by far, is Le Pressoir, on the northern end of the strip. Calmo's cafe is very nice right next door. American tourists probably won't know either of these places. Le pressoir is high end, Calmo's is mid priced. I agree with the LoLo suggestion, and Talk of the Town is the best. But there are great places on the Dutch side. Try Halsey's in Simpson Bay, and Bliss in Maho (At the caravanserai Resort) And yes, eat at the Marina. Tropicana is nice and so are most of the others on the water.
We ate at Il Nuttuno in Grand Case an excellent meal, some of the best calamari I've ever had. Be sure to have one of their aged Grand Marniers after dinner. We had heard a lot about Spiga but is was closed when we were there. Also the french side of St Maartin is great a number of good restaurants around the marina.
It's hard to find a bad meal in Grand Case. Try lots of places but my recommendations are:
1. Expensive - Bistro Caribe - excellent, excellent, excellent!
2. Cheap - the lolo's(outdoor places on the pier) - try Talk of the Town
3. Venture out to the rest of St. Maartin but eat mainly on the French side.