On the above, you have posted one article about waterboarding CIVILIANS which is indeed a war crime (as is waterboarding a uniformed combatant). Both are against the Geneva Convention. But that article states that those breaking the GC in this way were sentenced to 15 years hard labor (which is ALSO illegal to do to civilians in wartime, BTW). This contradicts McCain's statement and says nothing about whether the use of waterboarding on non-uniformed combatants is legal or not.
The second Article is contradicted by the first. McCain's statement states that Japanese captives were hung for waterboarding, this is a lie.
The third is a horribly befuddled Wiki entry in which they site a WaPo article about waterboarding that makes no mention of the guilty being hung, which again doesn't support McCain's statement that they hung Japanese for waterboarding.
What McCain has done is like arguing that a man who robs a convenience store, punches a customer and murders the clerk who is then executed was executed for punching someone. IT is a common tactic for those wanting to turn a logical legal argument into an emotional cluster-F where the truth is tertiary to emotion and political expedience.
Waterboarding is a crime by the Geneva Convention if used on protected classes of prisoners (civilians and uniformed combatants) and that is the reason some Japanese where sentenced to hard labor for water boarding. It is a conditional crime like many in the GC that is used to protect classes of POWs.