no i haven't but i take it you have, a guy i work with was a Navy Corpsman CWO4(at retirement and one of the first in the Navy to go through their PA program) and was at the SERE school during times it was done we talked at great length about this at the time and he says it's "Unpleasant and uncomfortable but not torture." i take him at his word
3 what threat of imminent death is there, dead men don't talk, and 4 who else is being threatened if they don't cooperate?
The threat of imminent death is not only experienced on the waterboard (there are known, serious medical events on the board), but also in SERE there are mock executions.
When another is being question while the primary is on the board, in the fitful throes of what seems like imminent death, that applies to #4.
Even ebing witness to such and event will burn in your memory forever. For those who participated, PTSD candidates all.
Nobody has ever applied the waterboard who has not been on it themselves. That's rule #1.