In a large pot, bring 4 quarts water, potatoes, ½ cup Old Bay, onion, garlic, and bay leaves to a boil over medium-high heat. Reduce heat to medium and cook until potatoes are just tender, 10 to 15 minutes. Add corn and sausage; cook until just tender, 5 to 7 minutes. Remove from heat and add shrimp; cover and let stand until shrimp are pink and fully cooked, 5 to 10 minutes. Drain shrimp mixture, reserving 1 cup cooking liquid.

In same pot, melt butter over medium heat. Remove from heat. Add shrimp mixture, reserved 1 cup cooking liquid, remaining 2 tablespoons Old Bay, and lemon zest; toss to combine. Serve with lemons. Garnish with parsley and Old Bay, if desired.