Prepare grill to medium-high heat. Score the skin side of the fish by cutting 2- to 3-inch-long, ¼-inch-deep marks along the length of the fillet, about 3 inches apart. Be careful not to cut all the way through the fish. Set aside.
In a medium bowl, whisk together mustard, 4 tablespoons olive oil, garlic, and zest and juice of the lemon and lime. Mix in lemon and lime wedges and cilantro. Set aside.
Rub both sides of the fish with remaining olive oil and season with salt and pepper. Place fish on grill and cook 1 to 2 minutes on each side to brown the exterior. Remove from grill. Lay a large piece of foil (twice the length of the fish) on a clean work surface. Spoon half the lemon and lime mixture onto one half of the foil. Lay fish flesh side down on top of mixture. Spoon remaining mixture onto the skin side. Fold the foil over the fish and seal edges.
Place foil package on grill and cook 4 to 5 minutes on each side. Let fish rest 2 minutes, then open foil (use caution with escaping steam). Place the fish on a large platter (or divide among six individual plates) and spoon Grilled Corn and Watermelon Relish over the top. Serve immediately.
Grilled Corn and Watermelon Relish
Preheat grill to medium-high heat.
Rub 3 tablespoons olive oil over corn and season with salt and pepper. Place on grill and cook for 8 to 10 minutes, turning frequently. Add watermelon to grill and cook for 1 to 2 minutes on each side. Remove corn and watermelon from grill and set aside to cool. Once cooled, cut corn kernels off cobs and dice watermelon into ½-inch cubes.
In a saucepan on the grill or stove top, heat maple syrup until bubbling. Add watermelon and stir to combine. Remove from heat and set aside.
In a bowl, combine corn, garlic, onions, cilantro, zest and juice of lemon and lime, remaining 1 tablespoon of olive oil, and maple syrup and watermelon. Season to taste with salt and pepper.
Makes about 4 cups.