Combine first 7 ingredients in a large bowl; cover and marinate in refrigerator 30 minutes.
Bring water to a boil in a medium saucepan; gradually stir in couscous. Remove from heat; cover and let stand 5 minutes. Fluff with a fork; cool. Add couscous and fresh basil to tomato mixture; toss gently. Garnish with basil leaves, if desired.