Thaw fish completely if frozen. Pat dry thoroughly with paper towels. Cut into serving-sized pieces if needed. Lightly dust each piece with flour.
In a bowl, whisk together flour, baking powder, salt, and pepper. Slowly add cold water (or beer), stirring until smooth. The batter should be slightly thin, similar to pancake batter.
Add oil to a deep skillet or pot (about 2–3 inches deep). Heat to 350–375°F.
Dip each floured piece into the batter, allowing excess to drip off.
Carefully place fish into the hot oil. Fry for 4–6 minutes, turning once, until golden brown and crispy.
Remove and place on paper towels or a wire rack. Sprinkle lightly with salt while hot. Serve immediately.
