Thanks for bringing this up. Millions of children swim safely every summer, but I recently witnessed a near-drowning and will say from that experience that it can happen very easily, even with careful adult supervision.
These articles might be helpful:
(the original news story)
This clarifies what happened and has some other helpful information for parents worried about drowning, most importantly that this boy experienced delayed/secondary drowning, which is far less common than true dry drowning.
The last two paragraphs of the snopes article provide lots of useful advice for parents.
The CDC also provides good info on preventing drowning:
Drowning is usually a silent occurrence, especially with young children - don't expect to hear splashing or struggling, they just go under and don't come up.