Prepare a gas grill or charcoal fire. In a 3 quart microwave safe casserole, combine the potatoes, onions, carrots, celery and water. Cover with microwave-safe plastic wrap. Microwave on full power for 5 to 6 minutes or until the vegetables are hot and just beginning to cook, stirring once or twice during cooking time. Drain, then stir in the salt and pepper. Place the potato mixture on a large sheet of heavy duty aluminum foil. Top with the bacon pieces and dot with butter or margarine. Bring up the ends of the foil to wrap securely and fold the top of the foil to seal. Place the wrapped bundle on the grill, 4" to 6" above medium-hot coals. Cook for 25 to 30 minutes or until the vegetables are tender.