What the Japanese did wasn't waterboarding, it involved putting a hose in the prisoner's mouth and forcing him to swallow a very large quantity of water or drown. And it was indeed one of the mildest things they did to prisoners, a surprisingly small number of the SOBs were actually executed, especially compared to the number who deserved it, and I don't believe any of them were executed exclusively for using this particular technique, but rather for killing prisoners in all sorts of ways to including this shit with the hose.