Thaw shrimp, if frozen. Peel and devein shrimp. Using a sharp knife, carefully slit the shrimp all along its inner curve, cutting just three-fourths of the way through the flesh. Do not cut through tail. Turn the shrimp cut side down. Using the flat side of a meat mallet, slightly flatten the shrimp. Stir together the sherry and the 1/2 teaspoon gingerroot. Toss with the shrimp. Cover and let stand while preparing oil and batter.
In a wok or 3-quart saucepan heat 1-1/2 to 2 inches cooking oil or shortening to 365 degree F. Meanwhile, for batter, in a small mixing bowl combine the flour, green onion, baking powder, salt, and the 1/4 teaspoon grated ginger. Add the water. Using a rotary beater or wire whisk, beat until smooth.
Pick up each shrimp by the tail. Dip all but the tip of the tail into the batter, swirling to coat. Fry shrimp, a few at a time, in the hot oil about 3 minutes or until golden, turning once. Using a wire strainer or slotted spoon, remove shrimp from oil. Drain on a wok rack or on paper towels. Keep warm in a 300 degree F oven while frying remaining shrimp. If desired, serve shrimp with Sweet-and-Sour Dipping Sauce. Makes 12 appetizer servings.