One may drink water, tea and coffee before davening, especially if it enables him to function properly during davening. Regarding sugar in the tea and coffee, the Mishna Berura[2] generally objects, but Rav Ovadya Yosef justifies the common practice of adding sugar.[3]
The question of hatarat nedarim is quite interesting. The Rama[4] says that even those who have the custom to fast on the day before Rosh Hashana may eat at a brit mila that day. The Magen Avraham[5] says that hatarat nedarim is not needed[6] and that this leniency can be extended to one who is mildly sick.[7] However, the Shulchan Aruch[8] states that one who wants to forgo this fast because he is not healthy needs hatarat nedarim, and the Rama’s silence implies that he agrees. Why does the Rama require hatarat nedarim for sickness but not for a brit mila? The Shach[9] claims that when people adopt the custom of fasting, they mentally exclude cases of brit mila but do not anticipate sickness.
The Dagul Merevavah[10] argues with the Shach’s explanation, as he considers it based on a difficult set of assumptions. He reconciles the apparently contradictory sources with the following distinction. One does not require hatarat nedarim to temporarily suspend a good minhag under specific circumstances, such as the case of the brit mila. He claims that the situation of the Shulchan Aruch[11] is different, as it concerns one who plans to stop his practice of fasting permanently because of a weakened constitution. The Dagul Merevavah implies that his thesis goes along the lines of the aforementioned Magen Avraham. The simple understanding of the Magen Avraham, however, is not identical with that of the Dagul Merevavah. The Magen Avraham differentiates between one who fasts due to a widespread practice and one who goes out of his way to accept a stringent practice. In the latter case, one requires hatarat nedarim, and the simple implication is that this applies even if he wants to stop the minhag temporarily.[12]
Based on the Dagul Merevavah, Rav Mordechai Eliyahu ruled that if you want to drink before davening only during the period of your shelichut, which presents a special situation, you would not require hatarat nedarim.