Shallow water blackout is a common cause of drowning. Lifeguards should be, and sometimes are, trained to be on the lookout for activities which leads to this event.

As I understand it, there are two common scenarios that often precede this. The first -- swimmers or others training -- is the one in the article you referred to. Many elite senior USS swimmers do "breath-holders". My 16 year old son does. The second -- elementary or middle school boys competing in breath holding or underwater swims -- was involved in the drowning I saw. (BTW, that boy did recover.)

In the first case, those training should NEVER do breath holders without someone 'guarding' attentively.

In the second, parents and lifeguards need to watch out for competition with unequally skilled kids. Male ego gets involved, and the drowning victim is often the weaker, and embarrassed about it, swimmer.

In BOTH cases, swimmers need to be encouraged (or with boys) prevented from hyper-ventilation. As I understand it, 1 or 2 deep breaths is fine: more are not!