1. Bring water to a boil; add shrimp, and cook 2 to 3 minutes or until shrimp turn pink. Drain and rinse with cold water. Peel shrimp; devein, if desired.
2. Layer shrimp and onion slices in an airtight container.
3. Whisk together oil and next 10 ingredients; pour over shrimp mixture. Cover and chill 24 hours, stirring occasionally.