I've been on vacation and went diving after a beer (unplanned dive I was invited to do, while I was laying around enjoying that beer) I wasn't impaired on that 25fsw beach dive, but in the future I don't plan to willingly drink and dive. It's just not a smart thing to do before diving, and the reasons have been beat to death.
Now as far as drinking after diving is concerned, I know some will have their theories about not drinking post dive, due to higher DCS susseptability, but my answer to that is bottoms up. Many boats I've been on, guys bring a 6pk for the ride back to the dock. Plus there's nothing more relaxing than soaking in the hot tub with a cold brew after a day of diving. ( I know, I'm gonna get bent, but seriously, it's the best.)