In a 12-quart stockpot, combine water, Old Bay seasoning and salt. Bring to a boil. Add potatoes, onion, lemon and garlic; cook for 8 minutes. Add smoked sausage and cook for 3 minutes. Add corn and cook for 7 minutes.
Add shrimp and mushrooms; cook until shrimp turn pink, about 4 minutes.
Drain shrimp, sausage and vegetables in a large metal colander and then transfer to a large serving plate. Garnish with lemon wedges and sprinkle with chopped parsley and additional Old Bay seasoning, if desired.