Cook salt pork in a large Dutch oven until browned and crisp; discard pork. Sauté onion in pork drippings until tender. Add fish, potatoes, and water. Bring to a boil. Reduce heat; cover and simmer 20 minutes. Gradually add milk, salt, and pepper, stirring constantly. Cook over medium heat 10 minutes or until thoroughly heated. (Do not boil.)
Place 1 English muffin half in each soup bowl. Spoon chowder over muffin. Serve immediately.