Alcohol is a diuretic. It causes your body to remove fluids from your blood through your renal system, which includes the kidneys, ureters, and bladder, at a much quicker rate than other liquids. If you don't drink enough water with alcohol, you can become dehydrated quickly.
Yes, reaching for a beer after a dive is not a good idea. There are a couple of theories why.
The first one is that goes hand in hand with the common scuba diving problem of dehydration.
Alcohol further dehydrates you which interferes with the release of nitrogen from your body. Some say that dehydration is one of the biggest contributors to DCS and alcohol aids that.
The first drinks that we should be talking after scuba diving are those that will rehydrate us. No alcohol or caffeine-based drinks.
The second issue is if you drink a lot. The alcohol can mask the symptoms of DCS and thus delaying treatment if you do have DCS.
Things you cannot do straight after scuba diving · Aussie Divers Phuket