There may be other reasons to prefer beer where the alcohol is relevant of course, just not for freshness. And freshness could absolutely have been relevant in the choice of drinks to load, together with low cost and acceptance in general.
> There may be other reasons to prefer beer where the alcohol is relevant of course
I imagine these are largely the same reasons people drink beer today. Spoiler: it generally ain't hydration or avoidance of disease.
That's what I was thinking. Why would boiled water kept hermetically sealed in a glass bottle for a year go bad ?
Keeping water fresh is not quite as difficult as you might think. For one thing, wood has naturally antibacterial properties; it can be trivially sealed with pitch and tar (which also has antibacterial properties), and it just takes one quartermaster to babysit it.
If anything, beer is a way of preserving calories and boosting morale. The fact that drinking a gallon of it translates roughly to drinking 97% of a gallon of water and does end up being quite hydrating just doesn't imply that people didn't also drink water sans beer.