Melt margarine in a Dutch oven over medium heat. Add onion, celery, bell pepper, and garlic; sauté 8 minutes or until tender. Sprinkle onion mixture with flour; stir well. Cook 1 minute, stirring constantly. Add potatoes and next 6 ingredients (potatoes through water); stir well. Bring to a boil; cover, reduce heat, and simmer 20 minutes or until potatoes are tender.
Add scallops; cover and cook 4 minutes or until scallops are done. Sprinkle with parsley.