Bring 5 quarts of water to a boil in a 10- to 12-quart pot. Squeeze juice from the lemons into water, then add the halves. Add onions, garlic, and spice bundle.
Add potatoes to pot, reduce heat, and simmer, covered, until potatoes are almost tender, about 10 minutes. Add corn; continue cooking 5 minutes more.
Stir in shrimp, submerging them completely. Cover partially, and let stand until shrimp are cooked through, about 3 minutes. Ladle 2 cups of broth into a glass measuring cup; drain the shrimp and vegetables in a large colander, discarding remaining liquid.
Make spicy butter: In a small saucepan, stir together melted butter and hot sauce to taste. Divide butter among small dishes or ramekins.
Divide reserved broth among small bowls for dipping. Transfer shrimp and vegetables to a large platter, and serve with spicy butter and lemon wedges, if desired.