Bring the water to a boil in a large pot. Squeeze the juice from the lemon into the water, and throw in the halves. Add onion, garlic, salt and crab boil. Reduce heat to medium-low, cover, and simmer for 10 minutes.
Add the potatoes and sausage, and return to a boil. Simmer covered for 20 minutes. Break the ears of corn in half, and add them to the pot; cover, and cook for 10 more minutes. Remove from the heat, stir in the shrimp, and cover for 5 minutes. Drain off liquid before serving. It may be reserved for other uses such as soup stock.
Stir together the melted butter and hot sauce. Serve with the seafood and vegetables for dipping.