In a microwave-safe dish, combine potatoes and margarine. Cover and microwave on high for five minutes, or until heated through. In a small bowl, combine egg whites, water, ham, and salt. Add to potatoes. Mix lightly. Cover and microwave on high for three minutes, or until edges are set. Move cooked portions to center. Cover and microwave again on high for one minute, or until center is set. Sprinkle with cheese and let stand five minutes before serving.