After some disappointment in ordering beer-battered fish at a local eatery, I battered my own last night. I cut up cod fillets into smaller pieces, roughly all about the same thickness, and dipped them in the batter below. This batter also yields awesome onion rings, and I've modified it in the past to do finger steaks, too. Use a deep-fryer, or 3-4 inches of oil in a heavy duty pan, heated to 375 degrees. A mixture of peanut and vegetable oil works nicely.
3/4 cup flour
3/4 cup cornstarch
1/2 tsp baking soda
1 tablespoon of powdered sugar
then season to taste with white pepper, granulated garlic, paprika (or maybe seasoned salt instead of all of that)
1/2 cup of water
1/2 cup maybe more, of dark beer or hearty ale
Whisk everything together to make a stiff batter - maybe add a bit more ale, if needed. Pat fish dry, dip in batter and drop in oil. The fish cooks in 3-5 minutes. When the outside is golden brown, they're done.