Very interesting! Could easily be learned rather than genetic - but I would bet on a mixture of genetic aptitude and training. I wonder whether they also have other aquatic adaptations (thicker layer of fat under the skin and less body hair, for example)?
Apropos of which, is anyone familiar with the hypothesis that humans went through a semi-aquatic phase at some point during their evolution (as outlined by Elaine Morgan in her book 'The Aquatic Ape', among other places)? The theory has been kicking around since the late 1920s but seems to lack mainstream support, though I find the evidence pretty compelling -

Aquatic Ape Theory doesn't deserve much of your time.

The evidence for the aquatic ape theory is in our anatomy. Anyone who ignores such evidence is a fantasist.

- the hair that we do have on our bodies goes in the direction that's ideal for swimming, rather than straight down as in other apes

- we have subcutaneous fat (fat just beneath the skin and above the muscles). only aquatic mammals have this

- the closest thing to our upright posture in other apes is the proboscis monkey which spends much of it's time wading through water

- unlike all other land mammals, infant humans have an automatic diving reflex which prevents drowning

- again, unlike all other land mammals, human beings can consciously control their breathing - necessary for diving and swimming