Q. Is there an Isur of Bishul Akum on water?
If not, if my gentile butler boils himself a kettleful of water on Shabbos, may I drink what he leaves in the kettle after he’s taken whatever he needed for himself?

A. The prohibition of Bishul Akum (food cooked by a Gentile) does not apply to water, since it can be consumed without cooking and does not change when heated. (Avoda Zara 37b – 38a, Shulchan Aruch Y.D. 113: 1. See Teshuvo Vehanhogos 4: 173, in regards to coffee and tea prepared with such water.)
In regards to water that was heated by a Gentile on Shabbos, if he prepared them for the use of an Yisroel, even if he was not requested to do so, but did so from his own free will, they would be prohibited until Motzei Shabbos. (Adding the time it would take to prepare them then or Bichdey Sheyasu, see OH. 365.)
If he heated the water for his own personal consumption and had some left over, many Poskim permit ingesting them in Shabbos.
Teshuvos Vehanhogos (V. 4 Leket Minhagim Mehagrach 36, p. 434) quotes that Rav Chaim Soloveitchik zt”l would permit drinking the left over water in Shabbos, however Rav Meir Simcha Hacohen (Ohr Sameach) zt”l was stringent, since they may be included in the decree of Anshei Tiberia (Shabbos 39a.) Teshuvos Vehanhogos writes that if the amount of water heated by the gentile was small and would not suffice for washing, but just drinking it should be permitted.
Horav Shlomo Miller’s Shlit”a opinion is that it depends also if the Gentile knows and is aware of the presence of this Yisroel (makiro) and would be adding more water for his sake (Mishna Berura 276: 9 – 10, regarding lighting a fire and 325: 66 addressing animal feed harvested by the Gentile.) If he does not know and is unaware of the presence of the Yisroel, the remaining water would be permitted.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a