Bring the water to a boil in a medium saucepan, and gradually stir in the couscous. Remove from heat; cover and let stand 5 minutes. Fluff with a fork. Combine couscous, turkey, celery, peas, and onions in a large bowl. Combine mayonnaise and remaining ingredients; stir well with a whisk. Add to couscous mixture; toss well. Cover and chill at least 2 hours.