Heat grill. In 3-quart microwave-safe casserole, combine potatoes, onions, carrots, celery and water. Cover with microwave-safe plastic wrap. Microwave on HIGH for 5 to 6 minutes or until vegetables are hot and have just begun to cook, stirring once during cooking. Drain. Stir in salt and pepper.
Cut 18x18-inch sheet of heavy-duty foil. Place potato mixture on foil. Top with bacon and dot with margarine. Wrap securely using double-fold seals, allowing room for heat expansion.
Immediately place on gas grill over medium-high heat or on charcoal grill 4 to 6 inches from medium-high coals. Cook 25 to 30 minutes or until vegetables are tender.
To serve, carefully open foil packet to allow steam to escape.