Stir together water, ketchup, soy sauce, cornstarch, honey, crushed red pepper, and ground ginger in a small bowl until combined. Set aside.
Heat oil in a large skillet over medium-high heat. Cook and stir green onions and garlic in hot oil until fragrant, about 30 seconds. Stir in cooked shrimp; toss to coat with oil. Pour in ketchup mixture. Cook and stir until sauce is bubbly and thickened.