In a saucepan, combine soy sauce, sherry, sugar, garlic, and ginger. Bring to boil over moderate heat. Allow marinade to cool. Pour over fish in a shallow glass pan or in a large zip bag.
Marinate about 30 minutes, turning several times. Cook on well-oiled grill, 10 minutes per inch of thickness and brush frequently with marinade. as a dipping sauce or pour over rice for good flavor.