Peel and devein shrimp. Bring 3 cups water to a boil in a saucepan; add shrimp, and cook 3 to 5 minutes or until shrimp turn pink. Drain well; rinse with cold water. Chill.
Bring remaining 2 1/4 cups water to a boil in a saucepan. Remove from heat. Add couscous; cover and let stand 10 minutes or until couscous is tender and liquid is absorbed. Fluff couscous with a fork, and transfer to a serving bowl. Add shrimp, crabmeat, and next 5 ingredients; toss well.
Combine lemon juice and next 4 ingredients; stir well with a wire whisk. Pour over couscous mixture; toss well. Cover and chill. To serve, place lettuce evenly on 6 plates. Spoon couscous mixture evenly over lettuce.