Were they trapped upside down, and somehow got into the airspace in their canoes?
I ask because when I was young and foolish I used to go sea kayaking. Part of the fun was "surfing" in to caves, but "breaking out" well before the cave wall.
It was a well known fact that if you miss-timed it, and got caught in the standing wave against the cave wall, you were as good as dead. You are trapped by the incomming waves, as well as being dashed against the wall. Your rescuers are helpless for the same reasons - they to will fall into the wave, capsize and drown, the force of the wave holding them under.
Strange story - lucky boys - rather foolish rescue attempt. (all due respect, but if it was similar circumstances they should have known better)