We nearly lost a kid at a b-day party last year because of exactly this. It's silent. It's easy not to notice. Almost happened to DS at a party once. Took him 3 years to have enough confidence to enjoy the water agsin and I'm not fully over it yet. Now, when I'm responsible for kids - any kids - someone I trust is ALWAYS on dry land with sharp eyes on the water at all times. Same goes for grown non-swimmers.