Just that split second before the act of drowning may have released Jack from this terror, he was yanked back up and left to dangle, dripping.

Only long enough to grab a few breaths, to pick up a bit of hope and then the machine shuddered once again and he was dropped into the murky water. This would continue for as long as Jack either remained conscious or continued to flop and flail about.

The intention wasn't to actually drown him (though it probably felt that way at times) but to see just how long, he'd continue to struggle against the inevitable evidence of his own failure.