I observed. So yes I saw it. Lasted about a minute. The type A individual that got selected from our POW group demonstrated very quickly how effective it was at getting people to comply. He would have taken physical beatings and not talked, but about 15 seconds of gasping for breath and he was ready to say whatever the instructors wanted him to say.
In 1978 I witnessed a man on the waterboard for no less than 5 minutes.
At least 45 seconds for each covering of the face.
At the end he was unconscious and had to be tended by the Doc. He returned later in the day but was quite timid and hyperventilating.