In that case; we're looking at two (?) direct impacts and one (?) indirect impact that alcohol consumption can have on a scuba diver underwater.
Firstly, that elevated amounts of alcohol in the bloodstream may have a physiological impact that renders them more, or less, susceptible to decompression illness. Gene_Hobbs' post seems to indicate that this won't be the case.
Secondly, that elevated amounts of alcohol in the bloodstream will have a physiological impact on the diver that inhibits their physical and mental capacity to a degree that their performance capacity is deteriorated to a level that constitutes 'unreasonable risk'. I don't believe that any agency or organisation has published a specific diving-related blood-alcohol limit. However, the study of performance at various blood-alcohol levels is well documented. Those studies have formed a basis for the imposition of laws regulating drinking and driving. Driving an automobile has a similar performance-risk relationship to scuba diving - so I'd propose that drinking/driving limits should be applied to scuba diving for that reason.
HOWEVER, I would be interested to know if the physiological impacts of alcohol have any known, or theorised, relationship with the manifestation of nitrogen narcosis. The issue of inert gas narcosis does differentiate scuba diving from other, non-diving, activities. This may well impact upon any blood-alcohol limits that can be tolerated when diving without causing unreasonable risk.
Thirdly, drinking alcohol produces a number of physiological side-effects besides inhibiting physical and mental performance. Several of those side-effects are also known pre-cursors to decompression illness. The primary one being dehydration.