Waterboarding is a war crime. That was settled in American law a good long time ago. Waterboarding is torture. Torture, in the context of armed conflict, is a war crime. George w. Bush has freely acknowledged that, despite repeated claims that "America doesn't torture," he did authorize the use of torture in the interrogation of al Qaeda suspects.
Bush claims the waterboarding disrupted al Qaeda plots and saved lives, a claim that is long disputed. In any case he's American and Americans are outside international law, not above it, just outside it as in "outlaw." That was their choice and they took it.