Combine tomatoes, mozzarella cheese, shallot, garlic, basil, balsamic vinegar, olive oil, salt, and black pepper in a large bowl. Refrigerate for 20 to 30 minutes.
Bring the water to a boil in a saucepan.
Remove from the heat, and stir in the couscous. Cover and let stand for 10 minutes; fluff with a fork. Allow couscous to cool, then toss with tomato mixture.