It’s well known that alcohol dehydrates you. How much alcohol (by volume) must a drink contain before the dehydrating effect of the alcohol outweighs the rehydrating effect of the water in it? For example, if you’re lost in the desert and suddenly come upon a bottle of vodka, are you better off drinking it or not? How about fortified wine? Regular wine? Beer?
I don’t know the scientific answer to your question.
However, living on the desert as I do I do have a practical answer. If you are lost without water you might as well drink the stuff because you are a goner anyway.
Sorry about that. Maybe someone will be along to give you what you are looking for.
Alcohol turns off a hormone called vasopressin (also known as ADH or antidiuretic hormone).
Normally, vasopressin acts to tell the kidney to hold on to water. When vasopressin is suppressed, as occurs when alcohol has been ingested, the result is that more water than normal is lost by the kidneys. Hence, the urine becomes inappropriately dilute. And, as a result, a person can become dehydrated.
In terms of how much alcohol causes what degree of vasopressin suppression, I don’t know. But still, it’s a reasonable guess that for relatively concentrated formulations of alcohol (e.g. high proof vodka), there is a net loss of water from the body as a result of the kidneys losing more water than was ingested in the form of the vodka (or whatever).
As this abstract notes, things are not entirely straightforward. The effect of alcohol to turn off vasopressin and thereby lead to water loss by the kidneys, is most pronounced as the alcohol level in the bloodstream is rising. Once a “steady-state” level has been reached, the consumption of more alcohol has less and less of an effect to suppress vasopressin. (I have to admit, I’m not clear how the authors of the abstract are using the term “steady state”)
My own experiments with alcoholic beverages suggest KarlGauss has it right and “things are not entirely straightforward.” 
After years of study, I now order a large glass of ice water with every scotch and I don’t order another drink until both glasses are empty. This not only helps me regulate my consumption, I’ve found it keeps me from waking up hungover the next day and feeling like my mouth could dessicate the Pacific Ocean. I’m convinced that nothing stops a hangover the morning after better than plenty of water the night before.
Aha! This answers something I always wondered about; why do you need the toilet more when you’re drinking? 
True. Drinking sufficient water will help hangovers, not only for your throat, but other symptoms also are from dehydration.
I guess this works if you don’t drink a lot of alcohol at one setting. It seems to me that what you are doing is flushing the electrolytes out of your system. You excrete more because of the alcohol and that liquid is replaced by fresh water.
Which is why you eat salty snacks when you drink.
When I was sitting an exam in wines and spirits a few years back, our instructor made a side comment that most wines are around the threshold of “one glass in, one glass out”. If that is the case then anything above 10-12% will cause you to lose a greater volume of fluid than you have imbibed.
Oh, and yes the practical part of the course was very enjoyable