Please read and share this post: Drowning Doesn't Look Like Drowning
The most important bit to know is this: People (and children especially) who are drowning are not yelling and not splashing. They don't look like they're drowning--unless you know what to look for.
And, as the article states, if your kids become quiet while swimming, get your ass over there and find out why.
Excerpt from the article:
There is very little splashing, no waving, and no yelling or calls for help of any kind. To get an idea of just how quiet and undramatic from the surface drowning can be, consider this: It is the number two cause of accidental death in children, age 15 and under (just behind vehicle accidents) – of the approximately 750 children who will drown next year, about 375 of them will do so within 25 yards of a parent or other adult. In ten percent of those drownings, the adult will actually watch them do it, having no idea it is happening (source: CDC).(emphasis added)