Preheat the oven to 400 degrees . In a medium bowl, whisk together 1/2 cup water with the lime juice, soy sauce, sugar, garlic, white and light green scallion slices, onion and red pepper flakes. Add the shrimp and marinate for 10 minutes.
Meanwhile, break the ramen noodles into small pieces and divide evenly among the centers of four 16-inch-long sheets of heavy-duty foil. Top with equal amounts of the shrimp mixture. Pull up the sides of the foil and pour 1/3 cup water over each of the shrimp-and-noodle mounds. Crimp the sides of the foil to enclose, and transfer the pouches to a rimmed baking sheet. Bake until the shrimp and noodles are cooked through, 8 to 10 minutes. Transfer to individual bowls and top with the dark green scallion slices.