Fill large stockpot with approximately 5 quarts of water, salt, shrimp boil, and cayenne pepper. Squeeze lemon quarters into water, then toss them in. Cover pot, and bring to boil. (Use your stove hood if you have one because the spices are strong.)
Stir together all ingredients for cocktail sauce and set aside.
Spread butter on cut sides of sourdough, and sprinkle with garlic powder and dried herbs. Bake at 350 degrees F for 10-12 minutes, or to your desired level of crispiness.
When water has come to a rolling boil, add potatoes. Cook 5 minutes.
Add corn, and boil 5 minutes.
Add shrimp, and continue boiling for 2-3 minutes. Turn off heat and cover. Let stand 5 minutes.
Carefully drain contents of pot in large colander. Remove potato and corn to serving bowl. Remove and discard lemon quarters and shrimp boil bag (if used). Rinse shrimp with cold water. (Skip rinsing and serve warm if preferred, but the cold water stops the cooking process and makes them easier to peel.)