Rinse the fish under cold water and remove any scales or visible bones. Soak in cold water for at least 12 hours, changing out the water a few times. Cut the fish into 2-inch pieces and set aside on a plate.
Add the flour to a large bowl. Using a whisk, slowly whisk in the water until a thick batter forms. You don’t want it to be runny, it should be thicker so that it can adhere to the fish without running off. Season the batter generously with salt and freshly ground black pepper.
In a large skillet or frying pan, heat the oil over medium-high heat until a droplet of water sizzles and jumps when dropped in the pan. If you have an oil/candy thermometer, the temperature of the oil should be at least 350 F (175 C).
Dip the fish in the batter and carefully place it in the hot oil. Fry the fish until golden brown on each side then remove and drain on paper towels. Place the fish on a baking rack over a sheet pan in a warm oven to keep it warm and crisp until ready to serve.
Serve with the delicious garlic dip known as Skordalia and enjoy.