Melt margarine in saucepan. Stir in flour, salt, and pepper. Gradually stir in milk, cooking until smooth and thick. Stir in cheese. Place haddock fillets in metal bowl which will fit loosely in cooker. Pour cheese sauce over fillets. Cover bowl firmly with aluminum foil.
Pour water into cooker. Place bowl on rack in cooker. Close cover securely. Place pressure regulator on vent pipe and cook 5 minutes with pressure regulator rocking slowly. Cool cooker at once. Serves 6.