Fill a large bowl halfway with ice and water. Pour 4 qt. water into a Dutch oven; squeeze juice from lemon into Dutch oven. Stir in lemon halves, black peppercorns, bay leaves, and salt; bring to a boil over medium-high heat. Remove from heat; add shrimp. Cover and let stand 5 minutes or just until shrimp turn pink. Stir shrimp into ice water; let stand 10 minutes. Peel and devein shrimp.