Waterboarding is a technique used on our own special forces troops to prepare them for capture and to test their psychological fitness for the kinds of missions they experience. That is where the technique originates.
President Trump is also in charge of the special forces, so he IS being 'waterboarded' in that sense. We have never required the actual Commanded-in-Chief to exhibit the physical prowess of his most specialized troops before being considered 'worthy' to command them. The closest we ever came was George Washington who imposed harsh conditions upon himself in response to the experiences of his active troops in the field, spending Christmas, for example, outdoors in a cold tent with no more food than what was being provided to the troops.
He was an exceptional cat and we didn't obligate him to do any of that. Trump is following the policy that our enemies who think they deserve to end our way of life can prove it first by enduring what we endure. As you can plainly see, they do nothing but ***** and whine about it like scared children, which is why we are kicking their ***.