Serves 4 as a first course.
Preheat oven to 450°F.
Leaving shells intact, pull legs off shrimp and with a sharp knife halve shrimp lengthwise. Arrange shrimp, shell sides down, in one layer in a shallow baking pan. In a small bowl stir together chili powder and salt and sprinkle evenly over shrimp. Shrimp may be prepared up to this point 3 hours ahead and chilled, covered.
Discard stems from spinach and diagonally cut scallions into 1/4-inch-thick slices. Drizzle shrimp with lime juice and roast in middle of oven until just cooked through, about 5 minutes.
In a large nonstick skillet bring water with gingerroot to a simmer over moderate heat and add spinach. Cook spinach, stirring, until slightly wilted, about 20 seconds. Remove skillet from heat and immediately stir in scallions and oil until combined well. Season spinach with salt and pepper.
Mound spinach in center of each of 4 plates and arrange shrimp halves around it. Serve shrimp and spinach with lime wedges.