There's a restaurant in the marina, service is slow at best, don't be shy about asking when your meal is going to arrive if it's taking a long time. I've only eaten breakfast there and it's always been good once you get it.

If you want a local experience there's the Creole Shack in St. Georges, it's buffet style (what we would call cafeteria style), get there before 5:00 for the best selection; you'll get a main, 3 sides and a beer for 20EC.

B&B's Crabshack, located on the carenage gets good reviews, never eaten there myself as it's a little upscale.

You could take the bus from the marina (going to the right when you get to the main road) and go to Wall Street, which is at the roundabout by Spiceland Mall, food stands, soup lady, shark and bake, people selling beers and drinks. Gets busy on the weekends.

Umbrellas which is on Grand Anse Beach, has good food, great happy hour and sometimes live music on the weekends, take the bus to the same roundabout as above, walk towards the mall and you'll see a sign for Umbrellas at the cut through to the beach. Nice beach bar, popular with students from the university.

At Kilingo you're kind of isolated, they have a restaurant on site. We didn't care for it, ordered jerk chicken, got a bland boneless chicken breast and some tired vegetables for 60 EC. Rule of thumb, if they're serving chicken breasts it's a tourist joint.
There is a local place next door to Kilingo, but I can't remember if they're open for dinner.

You could also go to either Prickly Bay Marina or the Dodgy Dock at True Blue, both are popular with yachties.