This is a hoary chestnut indeed which has been used for quite some time by many AAT/H proponents (including Hardy, Morgan, and Verhaegen). This time they usually actually give the reference: "M. McGraw, Journal of Pediatrics, 1939:485-490.", so one might assume they've read this research paper and know what it says. They always seem to mention the human infants and how their movements are usually "rhythmical and organized" and are "ordinarily sufficiently forceful to propel the baby a short distance through the water". So far so good. But they don't seem to ever mention the fact that the same study looked at other mammalian infants (opossum, rat, kitten, rabbit, guinea pig, and rhesus monkey) and found that they behaved the same way: "these rhythmical movements of the human infant are quite similar to those of other young quadrupeds in water".

AAT/H proponents consistently report only the info about human infants, and state that they react in a unique manner, ignoring the contrary facts the study reports regarding non-human infants, even though the info about both human and non-human infants is reported on the same page. Coincidence?

Note that an older chimpanzee was also tested and, just like older human infants, was inactive when placed in the water. Note too that this study found that "at no time did any baby show himself capable of raising his head above the water level for the purpose of breathing". So that's the sad secret of the drowning infants; what we actually find here is not so much "swimming babies" as infant mammals slowly drowning without a struggle.