Peel and devein shrimp. Bring water to a boil in a saucepan; add shrimp, and cook 3 to 5 minutes or until shrimp turn pink. Drain well, and rinse with cold water.
Combine shrimp, orzo, and next 3 ingredients in a large bowl. Combine lemon juice, oil, and salt; add to shrimp mixture, and toss lightly. Cover and chill at least 8 hours.
To serve, add chopped cucumber and tomato to shrimp mixture; toss well. Place 3 lettuce leaves on each of 5 salad plates. Spoon shrimp mixture evenly over lettuce. Garnish with cucumber slices, if desired.