Pour 2-inches of vegetable oil into a large skillet. Place skillet over large burner and heat oil over medium high heat. To check if the oil is hot enough, drop in 1-inch cube of white bread. The bread should brown in a 40 count.
Cut cod into 4 servings, 6 to 8-ounce portions and season with salt and pepper. Place a couple of scoops of flour in a pie tin. Combine egg and water in second tin. Season plain bread crumbs and mustard and cayenne in the third tin. Coat fish in flour, then egg, and then bread crumbs. Gently set coated fish into hot oil and fry 5 minutes on each side until medium golden brown in color.
When the fish is evenly golden all over, remove and drain on brown paper sacks.