From the DAN website FAQs:
https://www.daneurope.org/eng/faq1.htm
The individual who has alcohol on board may not feel impaired or even appear impaired to the observer, but that person definitely is impaired. This can persist for extended periods. In addition, alcohol produces dehydration, which is a major contributor to decompression illness. In any form, alcohol has a direct effect on the kidneys, causing an obligatory loss of body fluids. Recent discussions in scuba magazines, chat rooms and scuba forums have concluded that its OK to drink beer between dives during a surface interval. Some divers insist on drinking beer before and after their dives. Does drinking alcoholic beverages and diving pose any danger? The short answer is this: By drinking alcohol before and during diving trips, a diver severely endangers not only himself but his buddy! A study by Perrine, Mundt and Weiner found scuba diving performances significantly degraded at blood alcohol levels of 40 mg / dl (0.04 percent BAC). The study also cites a clear increase in the risk of injury at this level, which can be reached by a 81-kilogram man who ingests two 336-gram beers in one hour on an empty stomach. This study once again points out that one experiences a diminished awareness of cues and reduced inhibitions at relatively low levels of blood alcohol. Their study used well-trained divers who were being paid to do their best; their diving performances were being videotaped. Dr. Glen Egstrom, Ph.D., has stated the problem succinctly: his review of more than 150 studies on the effects of alcohol on performance has resulted in the following observations:
1. Ingestion of even small amounts of alcohol does not improve performance; to the contrary, it degrades performance.
2. While certain variables can speed up or delay the onset of the effects of alcohol, they are minor issues, which do not overcome the decrements to the central and peripheral nervous system.
3. Alcohol can be cleared from the blood at a predictable rate, usually .015 percent BAC per hour. This does not necessarily mean that the diminished performances have been completely eliminated in that time.
4. Alcohol, a depressant drug, slows certain body functions by depressing the entire central nervous system. Effects are noticeable after one drink.
5. The effects are mood elevation, mild euphoria, a sense of well being, slight dizziness and some impairment of judgment, self control, inhibitions and memory.
6. Increases in reaction time and decreases in coordination follow the dose / response curve quite well.
7. Alcohol is involved in roughly 50 percent of all accidents involving persons of drinking age.
8. Persons who have been drinking alcohol consistently underestimate its deleterious effects on performance.
9. Alcohol affects divided attention tasks to a greater degree than those tasks requiring a single focus of concentration. For example, a diver will be affected to a greater degree by a shallow water head-first dive, which required many interrelated decisions necessary to a successful dive than by lifting a heavy weight.