The Shulhan Aruch (Orah Haim 204:5) writes that nowadays, wine is not as sharp as in earlier times and thus it is important to be conscious of how much water is used to dilute it, lest it require the blessing of “Shehakol” and not “HaGefen”. The Rama (ibid.) adds on that as long as wine constitutes at leastone sixth of the mixture, its blessing would remain “HaGefen”. This is based on a teaching in the Gemara (Bava Batra 96b) that says that as long as the mixture contains a sixth of wine and tastes as such, its blessing would remain “HaGefen”. The Mishna Berura (O.H. 204:31) clarifies that the Rama is referring to actual wine that is being diluted, and not wine dregs, whose dilutionfactor is smaller because of their milder quality. Nevertheless, the Kaf HaHaim (O.H. 204), basing himself on the Peri Megadim, says that nowadays at leasthalf of the mixture should be wine. Rav Ovadia Yosef concurs with this approach. Practically speaking, as long as wine makes up at least half amixture and tastes like wine, the blessing of “HaGefen” could be recited.
The Gemara (Bava Batra 97b) also says that one who squeezes grapes recites “HaGefen” on the resultant juice. Although grape juice is not wine, it has the potential of fermenting and thus our Sages also included it in the blessing of “HaGefen”. Having said that, Rabbi Shlomo Zalman Auerbach and Rabbi Yosef Shalom Elyashiv maintain that since grape juice is weaker than wine, anydilution will nullify its Gefen status. Indeed the well known kosher grape juiceproducts such as Kedem and Welch’s are marketed co There is a product known as “Kedem Lite” which is marketed as having less sugar and fewer calories as the classic Kedem grape juice. There are some Poskim who say that its blessing should be “Shehakol” since it contains roughly 40% water, whereas others say that since the grapes used are more concentrated than conventional grapes, it would remain “HaGefen”. Since there is disagreement in the matter, one should avoid using Kedem Lite, for the Gefen, although onewho uses it does have on what to rely.
Summary: One could recite “Bore Peri HaGefen” on a mixture of wineand water that contains at least 50% wine. Any dilution of grape juice is renders the mixture questionable vis–a–vis reciting “HaGefen” and shouldbe avoided.