1. In a saucepan, bring water to a boil. Stir in couscous.
2. Remove from the heat; cover and let stand for 5 minutes. Fluff with a fork and cool to room temperature.
3. In a large bowl, combine the couscous, cucumbers, red pepper and onions.
4. Whisk together the buttermilk, yogurt, dill, vinegar, oil, salt and pepper. Pour over couscous mixture.
5. Cover and refrigerate for at least 1 hour.