Infant swimming is the phenomenon of human babies and toddlers controlling their breath and moving themselves through water. It's called the bradycardic response. It's not true that babies are born with the ability to swim, though they have reflexes that make it look like they are. Babies aren't old enough to hold their breath intentionally or strong enough to keep their head above water.

Most, though not all,[1] infants will reflexively hold their breath when submerged to protect their airway and are able to survive immersion in water for short periods of time. Infants can also be taken to swimming lessons. Although this may be done to reduce their risk of drowning, the effects on drowning risk are not reliable.[2] It's extremely important to note that while babies can imitate swimming motions and reflexes due to human's evolutionary past, they are not yet physically capable of swimming. (source wikipedia).