Place water, salt, and shrimp boil in a large pot over high heat. Squeeze lemons into pot and add rinds along with onion and garlic. Bring to a boil and add the potatoes, sausage, and mushrooms. When potatoes are almost done, add corn and cook for 5 minutes. Remove corn and potatoes and set aside. Add shrimp, return to a boil, and cook 1 minute. Remove from heat and add ice; allow to soak for 10 minutes. Drain shrimp, pour onto newspapers, and serve hot.