In small bowl, mix mayonnaise, onion and pickle relish. Cover; refrigerate.
Meanwhile, in shallow dish, mix flour and salt. In another shallow dish, beat eggs and water with fork. Place crushed cereal in third shallow dish. Dip fish in flour, coating well; shake off excess. Dip floured fish in egg mixture, then in cereal, coating all sides completely. Place coated fish on ungreased cookie sheet.
In 12-inch nonstick skillet, heat oil over medium heat until hot. Keeping at least 1 inch between fish fillets and cooking in batches if needed, cook fish in oil 3 to 4 minutes on each side, turning once, until well browned and fish flakes easily with fork. Serve fish topped with sauce.