Saute onion in butter in large heavy soup kettle. Add water, potatoes, celery, parsley, pepper and seafood seasoning. Cook uncovered for 12 minutes until tender. Most of liquid will have evaporated. Sprinkle flour over potatoes and stir. Add crabmeat, evaporated milk and milk; simmer on very low heat for 30 minutes more until potatoes fall apart and thicken the soup.