What happened to little Johnny?
Most likely, Johnny swallowed some water while he was in the pool, and some of the water made it to his lungs. This water disrupted the ability of the lungs' natural surfactant, a compound that reduces the surface tension inside the lungs and allows a person to breathe. Without the surfactant, Johnny was unable to breathe properly while he was napping, leading to his unfortunate demise.
How to keep safe
Cases like these are uncommon, and there are as yet no exact statistics on it. The bottom line is that although dry drowning is a real risk, it can easily be averted following safe water practices. Hopefully, what happened to little Johnny will serve as a reminder to every mother and child who enjoys swimming, and hopefully, it won't be a reason for you to completely opt out of this truly enjoyable summer activity.