Heat a large saucepan over medium-high heat. Coat pan with cooking spray. Add shrimp to pan; sauté 3 minutes. Add 1/2 cup onions, water, seasoning, and corn; bring to a boil. Cover, reduce heat, and simmer 5 minutes.
Remove from heat; stir in half-and-half and salt. Cover; let stand 3 minutes. Sprinkle with 1/4 cup onions.