Further to this post, you may read the two sections of the item here by Robert K. Goldman, concerning the prohibition and the definition of torture, to see how clear the international treaties and conventions on torture are. The prohibitions are absolute and non-derogable - 'which means that they cannot be suspended for any reason, including war or any other emergency situation'. Further: 'the proscription against torture is also widely regarded as having attained the status of jus cogens, or a peremptory norm embodying a fundamental standard that no state can contravene'.
Yet, Alan Dershowitz writes that 'the issue of "waterboarding" cannot be decided in the abstract' but only 'on a case-by-case basis'. (On which score see also this earlier post of mine.)
For a contrasting perspective to Dershowitz's, I recommend the article by Anthony Grafton on what we can learn about torture from the history of Renaissance Europe.