1. Preheat oven to 450°.
2. Combine egg white and water in a small bowl; beat well with a wire whisk. Combine breadcrumbs and pepper in a shallow dish. Dip fish pieces into egg white mixture, letting excess drip off. Roll fish pieces in breadcrumb mixture to coat.
3. Place fish on a baking sheet coated with cooking spray. Coat fish with cooking spray. Drizzle evenly with margarine. Bake at 450° for 12 to 14 minutes or until fish flakes easily when tested with a fork.
4. Combine mayonnaise and next 3 ingredients; stir mixture well. Serve fish with tartar sauce and, if desired, lemon wedges.