[phi-go]

I don't think babies can swim but they know not to try and breathe in water. Which is probably what you meant.

[mrtksn]

I think it's called "diving reflex", not very sure about it all but AFAIK babies can learn to swim properly quite early which makes me think that humans too come with a lot of "ready to go" features but maybe need some prompting to surface

[ghaff]

Kids (and even adults) definitely don't know how to swim off the bat though I have no doubt they could be taught earlier than many are. There's a reason some universities have a requirement to take swimming physical education absent a demonstrate ability to swim.