Peel shrimp, leaving tails on. Devein, if desired.
Bring 6 cups salted water to a boil; add shrimp, and cook 3 minutes or just until shrimp turn pink. Drain and rinse with cold water. Chill.
Combine sugar and next 5 ingredients in a shallow dish; add shrimp alternately with onion. Cover and chill at least 6 hours, turning often.
Drain shrimp, and discard marinade before serving.