So I think I've figured out why waterboarding is the USA's torture of choice.
It mirrors christian baptism.
As far as we know they don't pull out fingernails or drive bamboo slivers beneath them. They don't hang people from hooks embedded between their shoulderblades and beat them with canes. They don't lock them into Iron Maidens and tighten the screws.
They baptise them.
Makes perfect sense when you think about it.