In a desert where there is no water, can one take (steal) water from an idol worshipper causing him to very likely die, to be able to stay alive? I know the answer when one has no personal use for doing this but I am asking when one will not stay alive without it. (Like the way, lehavdil, that killing animals for nothing is not allowed, but it is allowed in order to eat them.)
Absolutely not. The Halacha is that one may not throw a non-Jew, even an idolater, into a pit, where he will eventually die (Avodah Zarah 26a, Shulchan Aruch YD 158:1, Tur YD 158, Rambam, Hilchos Avodah Zarah 10:1). (Yes, those Poskim understand that the Gemara refers specifically to the Seven Nations. However, if one is not allowed to actively kill one of the Seven Nations in this manner, where there is a commandment to kill them, certainly one should not be allowed to kill a non-Jew of a different nation, regarding whom there is no such commandment.)
The same should apply here: one cannot take water from a non-Jew, as that will result in his death, just like throwing him in a pit and letting him die there will.
Even when there’s a personal benefit - even to save your own life - seemingly it would still be forbidden, no different than directly killing someone would be. Murder is one of the few exceptions to the rule that anything goes to save a life (Sanhedrin 74a).