Remove shells from shrimp. De-vein and rinse. Pat dry on paper towels. Mix water, cornstarch, soy sauce and hoisin sauce in a small bowl. Set aside. Heat oil in wok; add ginger and garlic. Add shrimp and stir fry until they change to a pink color (about 5 minutes). Add scallions and stir fry for 1 minute; add sauce mixture and cook for 2 minutes.