A recent article from CBS states there is a condition called "dry drowning", this is when a child swallows a few drops of water from the pool/lake/ocean and goes down the wrong way into the lungs which causes the condition.

The top 2 ways to prevent this from happening is

1. Adult supervision

2. Learn how to swim 

Dry drowning has been a common case for kids and there are common signs from it:  

"They had a normal day and then they go to bed and they're coughing or they're wheezing or their parents see bubbling from the mouth," Dr. Maharam explained.

Of course, this is something that necessarily isn't a danger but it's something to be aware about. 

Read more here: http://www.cbsnews.com/news/kids-can-drown-hours-after-swimming-experts-say/