The episode of the BBC's long-running spy series, featuring the controversial interrogation technique, is slated to air Monday night.
The actor said that before he allowed himself to be subjected to waterboarding for the show he thought the technique was "a humane way of extracting information without hurting people."
He admitted he felt differently after he went through it himself, however.
"I realized that it really is a form of torture that shouldn't be used," Armitage told the Daily Telegraph. "I only lasted five to 10 seconds, and the sound of my voice crying out to stop isn't me acting. The psychological damage of doing that to someone for even a minute would be indescribable."
The show's producers told the Telegraph a health-and-safety adviser and medic were on set the day the scene was shot.
Armitage also stars in the TV adventure series "Robin Hood," in which his villainous Sir Guy of Gisborne killed Lady Marian at the end of the show's second season.