"In 1947 the US sentenced a Japanese officer to 15 years for waterboarding."

And the circumstances were what?

Was the man waterboarded a bona fide "Prisoner of War" as defined by the Geneva Convention?

Was he a recognised combatant? Wearing the uniform of a duly constituted and recognised power that was party to the conflict? Had the person being waterboarded fulfilling the above requirements been taken into capitivity? What was the subject of his interrogation?

My guess is that in the instance you are talking about the answers to the questions above would be as follows:

"Was he a recognised combatant?" - Yes he was

"Wearing the uniform of a duly constituted and recognised power that was party to the conflict?" - Yes he was

"Had the person being waterboarded fulfilling the above requirements been taken into capitivity?" - Yes he had been

"What was the subject of his interrogation?" - Anything other than anything relating to circumstances of his capture and the interrogation itself is illegal.

Now my guess is that in the last instance the "interrogation" had nothing whatsoever to do with the prisoners capture and it was just carried out to enforce "camp discipline" in which case the bastard who dished out the punishment deserved every single second of his 15 year sentence.

Now translate that to your Gitmo mob:
"Was he a recognised combatant?" - No he was not

"Wearing the uniform of a duly constituted and recognised power that was party to the conflict?" - No he was not

"Had the person being waterboarded fulfilling the above requirements been taken into capitivity?" - None of the above requirements had been met.

"What was the subject of his interrogation?" - Reasons surrounding his capture.

Squeeze the fucker til the pips squeak.