... It's not clear if the duo was following an official training program, or if they had devised their own workout, said Lt. Col. Robert Roy, head of Air Force recruiting in New York.
Either way, the military advises against certain breath-holding exercises or swimming underwater at length to avoid "shallow water blackout," which can lead to drowning.Very sad!
That reminds me. I wanted to link to this article: "Drowning Doesn’t Look Like Drowning." It begins with an unforgettable anecdote.