Imagine two identical people marooned on separate uninhabited islands. One has access to unlimited amounts of fresh water and absolutely nothing else. The other has the fresh water and also unlimited amounts of an alcoholic beverage containing no fat, carbohydrates or proteins. Since alcohol contains calories, could the second person live longer than the first, or would he have to drink enough that the negative effects of alcohol would outweigh the benefit of the calories?
So… are you volounteering?
Depends on the strength of drink. Some liquors dehydrate.
Really? I was under the impression that all alcohol dehydrates, on account of your liver using water to process the ethanol or something like that…
Alcohol is poisonous to us, our bodies are trying to dump it and its byproducts by excreting it via urine. Thirst is a natural response to many poisons.
That’s irrelevant. You have unlimited water.
The answer to the OP is an obvious yes. The negative effects of the alcohol won’t manifest until the other guy has long since starved to death.
But not all alcoholic drinks are pure alcohol. So it depends on whether the mix has enough hydrating molecules to pay off the dehydration of the alcohol molecules.
Even if all you had were evercleer and water, you can mix the two into a beer like ratio.
Not to mention that the second guy will enjoy his limited time on the island much more than the first. A water-only fast isn’t pleasant, so being able to drink yourself silly will help the time go by faster.
And when the other guy dies, you can eat him, too!
Any clues on how long? How long could an average healthy person live on nothing but alcohol and water?
Different islands. Otherwise the other guy would steal my booze. Can’t trust the bugger.
How is it obvious? Even if I accept your statement about negative effects as true… without carbs or anything else of value in the alcoholic beverage, do we get any POSITIVE effects out of it in this situation? 
I mean, not just psychological positives - would it actually help to stave off starvation? Can we metabolize alcohol into blood sugar?
While I’m the guy asking the question, the answer must be yes. Otherwise we wouldn’t say alcohol contains calories. We say that stuff we can’t digest doesn’t contain calories, even though our species may eat it and derive energy from it.
Alcohol is a carbohydrate, so it does have calories. This is essentially the same as asking if someone could live on sugar and water. Assuming an abundance of water, the alcohol guy would live longer than the water-only guy. Alcohol guy would probably die in a few months due to lack of protein, vitamins, minerals, and essential fatty acids.
Try his liver, first; it’s full of vitamins. 
That’s not a correct thing to say. By definition, the calorie is an indirect measurement of the energy we derive from food. To be precise, a dietary calorie is the quantity of heat required to raise the temperature of 1 kilogram of water by 1 degree celsius.
Alcohol provides calories because it is a carbohydrate. A pint of whiskey provides about half the energy you need for the day, but also some toxins that the liver has to eliminate. Other than energy in the form of carbs, it provides no nutrients.
Alcohol is a food, like protein, carbohydrates and fat, the body can convert it’s calories into energy. It has caloric value and it’s unused calories can be stored as fat.
Ethanol is C2H5OH, so if carbohydrates are compounds that consist only of carbon, hydrogen, and oxygen, then ethanol is a carbohydrate.