It's often recommended that you drink water before and during a heavy drinking session. Why is it that, despite being mostly water, beer dehydrates you?
Well, this question might get closed as a duplicate, but I'm going to answer it anyway, and try to answer it better. IMO, the suggested duplicate answers a different question, only answering the hydration question too, by side-effect. Personally, I like this question paired with that answer, better than that question.
tl;dr — Alcohol is a diuretic (defined: a substance that promotes the production of urine), causing you to expel more water than the body would normally expel.
That much has been shown conclusively.
If sufficient alcohol is ingested, the diuresis occurs at the expense of all cellular components, and dehydration ensues.
As far back as the 1930s, researchers were trying to understand whether alcohol—the substance itself—is a diuretic, or indirectly induces chemicals which are diuretic (or which inhibit antidiuretics) or triggers cortical functions that cause diuresis.
[Most] writers either state categorically or imply that alcohol per se has no diuretic action. In 1932, however, from a comparison of the diuretic effects of a given volume of water, with and without alcohol, [...] Murray concluded that alcohol itself was exerting a diuretic action. [...] The diuretic action of alcohol has now been demonstrated on five other subjects, and its mode of action investigated [...]
The first paper (1963) actually confirmed that alcohol—yes, ethanol itself—inhibits antidiuretic hormones, causing diuresis, causing dehydration. (Caffeine does this too, by the way.)
So, yeah. That's how that happens.
Unrelated, but I found this interesting in the second paper...
It [the diuresis] is initiated by the increase in blood-alcohol concentration and fails to be maintained if this concentration is kept steady, even at high levels [...] The diuretic response to alcohol differs markedly in one respect from that of the cerebral cortex. The latter is most affected by the rate of increase in blood-alcohol concentration: the greater this rate of increase, the greater the disturbance of function at any absolute concentration. The diuretic response, on the other hand, is dependent mainly on the duration of increasing blood-alcohol concentration and not on the rate of increase (Tables 6, 7). The naturally slow absorber, therefore, tends to give a larger diuretic response than the rapid absorber.
The slower you get drunk, the more dehydrated you'll be!