Preheat the oil to 350 degrees in either a deep fryer or a medium sized saucepan. In a medium bowl, mix together the flour, cornstarch, baking powder, salt, and cayenne pepper. Whisk in the water. The batter will get a little foamy, continue to mix. Pat the fish dry with a paper towel to remove excess moisture. Coat the fish in the batter, working to fully coat each fillet. Deep-fry the fish for 3 to 4 minutes. You want to cook the fish until it is golden brown. If you fry multiple pieces at once you may want to separate the pieces when cooking. Drain fish on a wire rack.