Wine in biblical times was wine, not unfermented grape juice. Unfermented grape juice is very difficult to keep without the aid of modern antiseptic precautions, and its preservation in the warm conditions of ancient Israel was not possible.
In fact, unless the process is inhibited, the juice from crushed grapes (“must”) begins to ferment naturally within 24-48 hours. Today, the fermentation process is usually regulated, but only in temperature-controlled tanks. Obviously, in Biblical Israel, no such “control” measures were available. Making wine (or vinegar) out of grape juice was the only way to preserve it. And, because of its unique pH (the measure of acidity or alkalinity of a solution) between 3.5-4.0 (7 being neutral) it is always potable (drinkable). The lowest pH that a food-born pathogen can tolerate is about 4.5 most can only survive a 6.
Finally, fermentation is the anaerobic conversion of sugar to carbon dioxide and ethyl alcohol by yeast. Therefore, the alcohol content of any given wine is (usually) determined by the amount of sugar present in the “must” at the time of processing—the higher the sugar content, the higher the potential alcohol. Most yeast cells (depending upon the variety) die when the alcohol content of the fermented “must” reaches between 15-16%. So, the fermentation process eventually stops naturally (unless it is artificially manipulated).
Was wine in Biblical times ever “cut” with water? Of course it was. But, was Biblical wine unfermented grape juice? No.