Bring water to a boil; add shrimp, and cook 3 to 5 minutes or until shrimp turn pink. Drain well; rinse with cold water. Chill. Peel and devein shrimp. Chop two-thirds of shrimp, leaving remaining shrimp whole. Cover and chill chopped and whole shrimp.
Beat cream cheese at medium speed of an electric mixer until creamy. Add sour cream, onion salt, and red pepper, beating until smooth. Spread mixture onto serving platter, shaping into a 5-inch circle. Cover and chill at least 30 minutes.
Combine chili sauce, Worcestershire sauce, lemon juice, and horseradish; stir well, and spread over cream cheese round. Sprinkle chopped shrimp over chili sauce mixture; top with whole shrimp. Garnish with fresh parsley sprigs and lemon slices, if desired.