Position rack in upper third of oven; preheat broiler.
Bring broth and water to a boil in a large saucepan over medium-high heat. Whisk in grits and half of the pepper. Reduce heat to medium-low, cover and cook, stirring occasionally, until thickened, 5 to 7 minutes. Remove from heat and stir in the cheese. Cover to keep warm.
Meanwhile toss the shrimp, scallions, oil, garlic powder, the remaining pepper and the salt in a bowl. Transfer to to a rimmed baking sheet. Broil, stirring once, until the shrimp are pink and just cooked through, 5 to 6 minutes. Serve the grits topped with the broiled shrimp and scallions.