I think Alex and Phaedrus are right, even if you didn't try to boil off some of the alcohol the beer would provide a net gain in water. Plus those complex carbs are golden in a survival situation. Regarding the carbs, they are even more important in the desert because when you digest carbs, there is a net release of free water that becomes available to the body. Protein digestion uses water in order to flush the tissues of the resulting ammonia while carb digestion provides more water to the body. Here is an excerpt on the topic from Golden & Tipton's excellent book which unfortunately costs a lot more in print than it used to.

Regarding caffeine & diuresis, IIRC much of the diuretic effect happens to people who don't regularly consume caffeine. Those who have caffeine regularly have less response to the mild diuretic effect.