Bring 3 qt. water to a boil; add shrimp, and cook 3 to 5 minutes or just until shrimp turn pink. Drain and rinse with cold water. Peel shrimp.
Combine shrimp and cocktail mix in a large zip-top plastic freezer bag. Seal bag, and chill 8 hours.
Remove shrimp from marinade; discard marinade. For a fun presentation, divide shrimp among martini glasses. Sprinkle each serving with chopped olives and capers. Garnish, if desired.