Bring water to a boil in a large Dutch oven. Add shrimp; cook 2 minutes or until done. Drain and rinse with cold water. Chill.
Combine onion and next 7 ingredients (onion through jalapeño) in a large bowl. Add shrimp; toss to coat.
Spoon 3/4 cup shrimp mixture into the center of each of 6 salad plates. Arrange 2 mango slices and 2 avocado slices spokelike around each serving. Garnish with cilantro sprigs, if desired.