Water When Thirsty
The Mishnah (Berachot 44a) states that if one drinks water when thirsty, one recites the “Shehakol Nihya Bidvaro” blessing. However, if one drinks water and is not thirsty, no blessing is recited.
For instance, if one is not currently thirsty and is only drinking water to swallow a pill or to clear one’s throat, one would not recite the Shehakol blessing.
Other Beverages
The Poskim quote the above as Halacha only with regards to water. However, if one drinks other beverages, such as orange juice, even if one is not thirsty, one must recite the Shehakol blessing since one ultimately enjoys the taste.
Hydrating Before a Fast
The author of Responsa Devar Mishpat (Ch. 4) writes that if one drinks a lot of water before the onset of Tisha Be’av or Yom Kippur, even if one is not currently thirsty, there is room to posit that one should recite the Shehakol blessing since the water one is drinking now will save him from being thirsty at a later time. He proceeds to support his logic with several proofs.
Nevertheless, Maran Rabbeinu Ovadia Yosef zt”l (Halichot Olam, Vol. 1, p. 352) disagrees and rules that one who drinks water before a fast in order to remain hydrated the next day may not recite the Shehakol blessing since at the moment, one is not actually thirsty.
Drinking Water on a Hot Day
Similarly, Maran zt”l writes that if one drinks water on a hot day to stay hydrated although one is not actually thirsty, one should not recite the Shehakol blessing before drinking.
The same applies to one who drinks a lot of water to avoid getting headaches.
Summary: One should not recite the Shehakol blessing before drinking water unless one is thirsty.
