Peel and devein shrimp. Combine water, lemon half, and bay leaf in a Dutch oven; bring to a boil. Add shrimp, and cook 3 to 5 minutes or until shrimp turns pink. Drain well; rinse with cold water. Chill. Discard lemon half and bay leaf.
Cook orzo according to package instructions, omitting salt and fat. Drain; rinse with cold water, and drain.
Combine shrimp, orzo, cucumber, and next 7 ingredients in a bowl; toss gently. Cover and chill thoroughly.
Place 1 cup shredded lettuce on each of 2 salad plates. Spoon chilled shrimp mixture evenly over lettuce.