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.

Layer shrimp and onion in an airtight container. Combine oil and remaining ingredients. Pour over shrimp and onion. Cover and chill 24 hours, stirring occasionally. Drain before serving.