Preheat a grill to high. Toast the coriander seeds in a dry skillet over medium-high heat, tossing, about 3 minutes. Crush the seeds on a cutting board (the bottom of a measuring cup works well), then pulse with the parsley, cilantro, ginger, vegetable oil, 1/2 teaspoon salt and 1 tablespoon water in a food processor to make a coarse mixture. Toss the corn and tomatoes with 2 teaspoons of the herb mixture in a bowl and season with salt. Tear off four large sheets (about 16 inches long) of heavy-duty foil.
Place one piece of fish in the center of each. (If the ends of the fillets are thin, tuck them under.) Top with a spoonful of the remaining herb mixture and surround with the corn-tomato relish. Fold up the ends of the foil and seal into packets, leaving room inside for heat circulation. Grill the packets, covered, until the fish is just cooked through, 8 to 10 minutes. Carefully open the packets and slide the fish and vegetables onto plates. Top with more cilantro and serve with lime.
