Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Cut fish into bite-size pieces. In a large saucepan cook onion in hot margarine over medium heat about 5 minutes or until tender.
Stir in flour. Add water all at once; stir until smooth. Stir in the potato, celery, sweet pepper, bay leaf, salt, and black pepper. Bring just to boiling; reduce heat. Simmer, covered, for 15 to 20 minutes or until potato is nearly tender.