Rinse the whiting briefly under cold water and pat it completely dry. Pat the shrimp dry as well if using. Season lightly with salt and pepper.
In a large shallow bowl, combine the cornmeal, flour, black pepper, cayenne, salt, and garlic powder.
Pour enough oil into a heavy skillet for shallow frying and heat it to about 350°F (175°C). The oil should be hot but not smoking.
Dredge each whiting fillet in the cornmeal mixture and shake off the excess.
Fry the fish in batches without crowding the skillet, about 4–5 minutes per side, until golden, crisp, and opaque inside.
Coat the shrimp in the same mixture and fry for about 2–3 minutes per side, until golden and opaque. Do not overcook.
Transfer the fried seafood to a wire rack or paper towels to drain. Serve hot.