I am sure this thread will generate a lot of traffic. Here goes my thoughts. I have not drank any alcoholic beverages before any dives. Plus, I will not dive with anyone who had a drink before diving. However, I have drank more than my share the night before. Not enough to have a hangover the next day. I never thought my skills were impaired by drinking the night before, I suspect I might be a bit niave.
I have posed this question to DAN. I suggest you do the same. You may come to a different answer than I. Per Dan, there is no direct cause between drinking and DCS. However, it is thought that drinking shortly after a dive MIGHT increase the chances of DCS due to increased dehydration and reduced blood flow in the body at a time when you are expecting to off-gas nitrogen. I have talked with instructors who feel drinking after diving is like drinking poison. Particularly challenging for resorts that offer wine/beer as you end you dives for the day. He made a big stink challenging the dive operations decision.
Although DCS might be one concern, I feel that making a mistake underwater due to alcohol consumption is a bigger concern, perhaps a Huge concern. Toward that end, I feel drinking before a dive is plain stupid. I am an EMT. I dive in the North East ocean under poor vis conditions. I always look for a challenge but dealing with a panicked diver because he/she ghad a drink before the dive is something I do not want to deal with. I can wait until the dive ends to have a drink.
Hope this helps - Tom