Combine pina colada mix, 1/2 cup water, salt and pepper.
Pour half of marinade into 11 x 7 x 2-inch baking dish. Place tuna in dish; pour remaining marinade over tuna. Cover. Refrigerate 4 hours, turning once.
Place tuna on broiler-pan rack. Discard marinade.
Broil tuna 3 inches from heat source for 3 minutes. Turn tuna over. Sprinkle with scallion. Broil 3 to 4 minutes or until desired doneness. Makes 4 servings.