1Spray 7-quart slow cooker with cooking spray. Place potatoes in slow cooker. Add water, beer, seafood seasoning, celery and onion. Squeeze lemons over mixture in slow cooker; add lemon halves to slow cooker.
2Cover; cook on Low heat setting 3 hours. Add sausage and corn. Cover; cook 2 hours longer.
3Increase heat setting to High. Add shrimp to slow cooker; stir gently. Cover; cook 15 minutes or until shrimp are pink. Turn off slow cooker; let stand 15 minutes. Drain. Serve with cocktail sauce.