The Shulhan Aruch (Orah Haim 254:4) rules that a fruit which is edible raw may be placed alongside a pot on the fire before Shabbat so it is cooked for the Shabbat meal. The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) explains that the Shulhan Aruch actually allows placing such fruit directly over a fire on Shabbat, and the Shulhan Aruch speaks of placing it alongside a pot only because this was the way fruits were heated in the past.
This Halacha marks an exception to the standard rule which forbids leaving raw food over an open flame on Shabbat, given the concern that one may stoke the coals, or adjust the fire, in order to accelerate the cooking. Placing uncooked food on an open flame before Shabbat is permissible only if it is half-cooked by the time Shabbat begins, in which case one is unlikely to try to accelerate the cooking on Shabbat. Why, then, does the Shulhan Aruch allow placing raw fruit on a fire before Shabbat?
The Mishna Berura explains that if a food is edible raw, there is no concern that one will take measures to accelerate the cooking, because the food does not have to be cooked at all in order to be eaten and enjoyed. Since one will not be pressured to accelerate the cooking process in such a case, a food which can be eaten without cooking may be placed on an open flame before Shabbat so it will be cooked for the Shabbat meal.
At first glance, we might assume that this would apply to water, as well. After all, water can and often is drunk "raw" – without being heated. Seemingly, then, it should be permissible to place a pot of cold water over an open flame before Shabbat so one can have hot water for tea after the Friday night meal.
In truth, however, this is incorrect. Several passages later (254:9), the Shulhan Aruch rules that one may not place cold water on fire before Shabbat. If one did place cold water on the fire before Shabbat, he adds, then the water may not be drunk on Shabbat (and even remains forbidden for a period of time after Shabbat). The Mishna Berura explains that since hot water is much more desirable than cold water, we are concerned that one might try to accelerate the heating process, and thus even though water can be drunk cold, one may not place cold water directly on the fire before Shabbat. (It must be emphasized that we refer here to an open flame, such as a stove without a "blech.")
Accordingly, the Mishna Berura writes that if one wants to place water over the fire before Shabbat for his Friday night meal, he may do so only if he allows enough time for the water to become hot by the time Shabbat begins. Rabbi Moshe Halevi (Israel, 1961-2001), in his Menuhat Ahaba, discusses the question of precisely how hot the water must be when Shabbat begins, and he distinguishes between two cases. If the water had been previously brought to the boiling point (100 degrees Centigrade, 212 degrees Fahrenheit), then it may be left on the fire before Shabbat even if it has since cooled, as long as it reaches at least 50 degrees Centigrade (122 degrees Fahrenheit) by the time Shabbat begins. If, however, the water had never been previously boiled, then it may not be left on the fire before Shabbat unless it will reach the boiling point by the time Shabbat begins.
Summary: It is forbidden to leave water over an open flame before Shabbat to be used on Shabbat unless it reaches the boiling point by the time Shabbat begins. If the water had previously been boiled, then it may be left on an open flame even if it has since cooled, as long as it will reach a temperature of 50 degree Centigrade (122 degrees Fahrenheit) by the time Shabbat begins.
