Gather ingredients.
Whisk flour, milk, water, baking powder, and salt together in a bowl until smooth.
Heat oil in a deep fryer to 365 degrees F (185 degrees C)
Dip fish fillets into the batter to coat.
Carefully lower fillets, one at a time, into the hot oil. Fry several fillets at a time, turning once, until cooked through and golden brown, about 2 minutes per side.
Drain on paper towels.
