Also, when kids (and especially babies) are drowning, it's usually not obvious. In adults when carbon dioxide levels increase past a threshold a lot of physically obvious manifestations begin, such as thrashing, driven by the autonomic system. And before then there'll often be coughing and other signs of distress, autonomic or otherwise. In small children this doesn't happen. Countless parents have literally watched their child drown before their very own eyes. A coworker almost lost a kid this way--the instant the kid accidentally inhaled water while playing in a hot tub, they immediately went lights out. Fortunately their other, older child was also playing in the hot tub and attentive. The parents had both briefly gone inside, proving the warning that it only takes seconds for tragedy to unfold.
I'm synthesizing here, but AFAICT a catatonic-like state is typical of small children in severe distress. (Anybody remember the Moth Radio Hour story about the man whose kid immediately went catatonic when they were surprised by burglars? IIRC, he was told this response was common of young children.) So if a baby suddenly inhales a large amount of water, they may immediately go from normal to non-responsive (possibly retaining muscle tone?), even though they're not yet physiologically drowning, and even though with lesser amounts of water they would normally cough and cry. This [lack of] behavior may be related, if indirectly, to the phenomenon of SIDS. An infant's physiology hasn't developed the various mechanisms to jump start respiration when something goes awry. (One unproven hypothesis behind SIDS is that an infant's breathing is partly moderated and even induced by their caretaker's breathing, such that if they can't hear, feel, or otherwise sense--some theories implicate carbon dioxide levels--another's breathing their own rhythm may be disrupted, or if disrupted less likely to resume. Thus co-sleeping may be better for infants, reducing SIDS risk, ceteris paribus--i.e. absent counter-indications, such as a parent who smokes or drinks.)
So with babies not only do you have to watch them to make sure they aren't presently, visibly drowning, you have to make sure (to some reasonable, mentally healthy degree) they couldn't possibly be drowning--i.e. they didn't or couldn't have inhaled water; that they're active, attentive, etc.
I'm synthesizing here, but AFAICT a catatonic-like state is typical of small children in severe distress. (Anybody remember the Moth Radio Hour story about the man whose kid immediately went catatonic when they were surprised by burglars? IIRC, he was told this response was common of young children.) So if a baby suddenly inhales a large amount of water, they may immediately go from normal to non-responsive (possibly retaining muscle tone?), even though they're not yet physiologically drowning, and even though with lesser amounts of water they would normally cough and cry. This [lack of] behavior may be related, if indirectly, to the phenomenon of SIDS. An infant's physiology hasn't developed the various mechanisms to jump start respiration when something goes awry. (One unproven hypothesis behind SIDS is that an infant's breathing is partly moderated and even induced by their caretaker's breathing, such that if they can't hear, feel, or otherwise sense--some theories implicate carbon dioxide levels--another's breathing their own rhythm may be disrupted, or if disrupted less likely to resume. Thus co-sleeping may be better for infants, reducing SIDS risk, ceteris paribus--i.e. absent counter-indications, such as a parent who smokes or drinks.)
So with babies not only do you have to watch them to make sure they aren't presently, visibly drowning, you have to make sure (to some reasonable, mentally healthy degree) they couldn't possibly be drowning--i.e. they didn't or couldn't have inhaled water; that they're active, attentive, etc.