Funny question, since I found out not too long ago in medieval times most people drank beer because the water was usually so bad and caused so many illnesses, even the children. I guess they must have been pretty happy, but I wonder about the hangovers.
I wonder why you say wine is the only water substitute? I guess the answer is in the question somehow. As Bill mentions the diuretic effects of alcohol, it's hard for me to imagine it as a water substitute, as well as it's hydrophilic or dehydrating effects, it seems counter-intuitive to be used as a substitute. As the medieval period it's another story for health due to pollution, contamination, etc. I always thought beer would be good because of it being a protein source and perhaps less alcohol than wines (less neural damage)

, but then wine does get better with age sometimes, doesn't it?
I tried checking your link, but it seems to be dead. I know water is in bad shape now, and will be worse in the future, but you could always filter or distill which is probably a better idea considering the water used to make the wine may still be in bad shape and lead to vinegar production. The only problem with distillation is the energy source (solar maybe?).
I like that the Romans distilled the wine for easier shipping, thus brandy was born

, and we all like Brandy right

They mixed in salt water with it also for a tasty treat, but for survival, I just can't picture it. So, I'm a bit curious about your source, it sounds a bit fanatical to use wine.
As for equivalent, well...it could be answered many ways as you've probably found out. Not just the same amount of water in the wine, but the loss of water from alcohol bonding and dehydration might be considered, if it's a question of hydration for the body. Perhaps the wine is considered a water substitute because of the denaturing effect of the alcohol on bacterial contamination, but then I'm on well water and I filter it also, so I don't worry about it too much. Eventually I suppose all the aquifers will be contaminated also from steam injection mining, or whatever, perhaps desalinization plants will come on line before huge global disaster

, I guess it's just a matter of cost, but nevertheless, good luck in your quest.