Place whitefish in a Dutch oven; add water to cover. Cook, uncovered, over medium heat, 8-10 minutes or until fish just begins to flake with a fork. With a slotted spoon, remove fish and break into bite-size pieces; set aside.
Pour cooking liquid into a 4-cup measuring cup. If necessary, add additional water to measure 4 cups; return to pan. Add potatoes, celery and onion; bring to a boil. Reduce heat; simmer, uncovered, 10-15 minutes or until vegetables are tender.
In a small bowl, whisk flour and milk until smooth. Stir into chowder. Bring to a boil, stirring constantly; cook and stir 1-2 minutes or until slightly thickened. Add clams, crab, shrimp, butter, pimientos, salt, pepper and reserved fish; heat through, stirring occasionally. Sprinkle with parsley.