Melt margarine in a large nonstick skillet over medium heat. Add cucumber, leek, and celery; cover, reduce heat to low, and cook 10 minutes or until vegetables are tender, stirring occasionally. Add broth, potato, water, salt, and pepper; bring to a boil. Cover, reduce heat, and simmer 10 minutes or until potato is tender.
Place cucumber mixture in a blender or food processor, and process until vegetables are finely chopped. Ladle soup into bowls, and top each serving with 1 tablespoon diced cucumber and 1 tablespoon yogurt.