In a 3-quart saucepan bring water and salt to a boil and stir in couscous. Immediately remove pan from heat and let couscous stand, covered, 5 minutes.
Cut medium tomato into quarters and in a blender puree with mustard and vinegar until smooth. With motor running add oil in a slow stream. Season dressing with salt and pepper. Halve the cherry tomatoes and toss them with the couscous and torn basil leaves. Dress with the vinaigrette and serve.