1. Preheat grill to 350º to 400º (medium-high) heat. Sprinkle grouper with pepper and 1/2 tsp. salt. Drizzle with 2 Tbsp. olive oil.
2. Grill fish, covered with grill lid, 3 to 4 minutes on each side or just until fish begins to flake when poked with the tip of a sharp knife and is opaque in center.
3. Combine chopped watermelon, next 5 ingredients, and remaining 1/2 tsp. salt and 1 Tbsp. olive oil. Serve with grilled fish.