Also of note is the statistic that FHN posted that said 75% of drownings occured when the children had been missing from sight for five minutes or less.
As a mother of four I've had ample experience with the fact that children operate at warp speed. You can literally turn your back for five seconds and in that time they can dismantle, climb to the top of, destroy, color on, or eat almost anything.
I'm a paranoid parent, and I still haven't managed to avoid them hurting themselves somehow. (Not badly thank goodness!)