Somewhat related, in terms of securing a baby for a life boating,
by making sure they are prepared in case they do fall in the water.
Children have swiming reflexes, such as holding breath, and moving limbs in swiming from motion from birth.
With regular practice they can incrementally improve their swimming abilities,
initial stages are conditioning, such as pouring water over the face, to trigger the breath holding reflex.
By 6-24 months children can be trained to flip themselves on their back if they fall in the water,
as well as do several seconds swiming, by 3 they can alternate to get to shore.
I think it's best to teach the kids personally,
so they can learn at their own pace with the loving care of their parents.
In terms of "cold-shock" as someone was mentioning about cold water,
this is something that is also completely dependentent on practice and conditioning.
For instance since I was an adolescent my friends and I would brake holes in the ice and go for dips in the winter.
even swiming when it's only a thin layer that can be broken easily.
Though to be safe it's best to keep winter-swims to under a minute,
a properly conditioned person could have fine motor function even for 10 minutes,
and swim as long as 30 minutes even in freezing water with limb movement.
Children have been known to recover from being hypothermacally unconscious for as long as an hour.
Main thing is to have rewarming after any hypothermic experience, preferably while dry,
if it is only mild, and the person is experienced, can passively raise own temperature,
if not, then may need active external heating such as other people, or sauna.
In case of severe hyopthermia (unconsciousness) active internal heating,
such as hot humid air like cpr could work, in combination with others.
When my friends when for dips into the salty pool,
we would run back to the sauna to heat up afterwards.
For good self-passive heating, it's important to eat fats,
especially unsaturated fats like those in nuts and seeds.
Tumo "breath of fire" a rapid breathing meditation is also effective for rewarming.
In terms of physical conditioning, can do so with cool, and then cold showers,
which are healthy, improve immune and organ function anyways,
and lower the chances of you getting cold from a breeze.
I know some parents that did cold showers with their kids,
daily since they were very young till they did it themselves,
and managed to avoid most (possibly all) childhood diseases.
they did it based on Porphyry Ivanov's teachings, who recomends ice cold water
Also note that the mammalian diving reflex is triggered by water under 21Celsius (70f) on the face.
So if doing water birth, it may be a good idea to have relatively (to western hot baths) chilly water,
If doing water conditioning that range would also be most useful for actually triggering the reflex.
edit to add:
note that water under 26 celcius for prolonged periods of time can induce hypothermia,
so likely some kind of compromise is best, likely over 26 and bellow body temperature (36).