Combine the flour, cornstarch, baking soda, baking powder, and salt in a mixing bowl.
Gradually add water while stirring until the batter is smooth. Let it rest for 10 minutes.
Prepare your fish, coating each piece evenly in the batter.
Heat oil in a frying pan or deep fryer to 375°F.
Fry the battered seafood until golden brown and crispy, turning as needed to cook evenly.
Drain the fried seafood on a wire rack to keep it crispy before serving.