Thaw fish, if frozen. Rinse fish; pat dry with paper towels. For marinade, in a shallow dish combine soy sauce, lime juice, ginger, and sesame oil. Add fish, turning to coat. Cover and marinate at room temperature for 30 minutes or in the refrigerator for 2 hours, turning fish occasionally. Drain fish, discarding marinade.
For a charcoal grill, arrange medium-hot coals around a drip pan. Test for medium heat above the pan. Place fish on lightly greased grill rack directly over the drip pan, tucking under any thin edges. Sprinkle fish with sesame seeds. Cover and grill until fish flakes easily with a fork. (Allow 15 to 18 minutes per 1/2-inch thickness of fish). Do not turn. (For a gas grill, preheat grill. Reduce heat to medium. Adjust for indirect cooking. Grill as above.) Makes 4 servings.