In the ancient Roman world, water wasn't too great. One common alternative employed by Roman soldiers was posca, a vinegar-based drink. They would make it from spoiled wine.
In the Gospels, the Roman soldiers gave Yeshua (Jesus) sour wine during his crucifixion. This might have been posca.
Posca kind of reminds me of lemonade: sweet, sour, and kind of refreshing. It takes some getting used to, though. The first sip seems like drinking salad dressing. But with a little time the taste can be acquired fairly easily.
For the authentic experience, I suppose you should use wine vinegar, although you can make similar drinks from balsamic or cider vinegar. Just don't use distilled vinegar.
Wikipedia suggests a proportion of 1/2 cup vinegar to 4 cups of water, but I found that to be far too strong. Here's what I recommend for 1 glass:
- 2 Tbsp vinegar
- 2 Tbsp honey
- 8-10 oz water
Some people recommend flavoring it with herbs like coriander. Salt is another common ingredient.