Waterboarding CLEARLY is torture. Whether it is a criminal matter is not clear to me. And I’m not sure a President can be prosecuted for actions taken while in office; the Constitution seems to provide impeachment as the remedy.
I’m also not sure what purpose such a prosecution would serve in this case.
I’m also not convinced that torture provides consistently useful information. Police detectives know that they can get criminals to confess under interrogation to all kinds of things that they did not do! That may be one reason why self-incriminating evidence is not permitted in a Jewish court.