They do have some reflexes related to swimming, in that they will hold their breath and work their feet, but those reflexes disappear with time and have to be re-learned.
It's fair to say that swimming is a learned practice.
Imho that isn't related to swimming. That's them trying to keep their heads above water by standing up, hopefully in relatively shallow water. Even with drowning adults, they don't so much try to swim as try to grasp onto something.
Compare dogs. They have a swimming instinct dedicated to movement. Dogs don't tread. They move forwards. We flail around trying to climb out of the water. They are trying to get to shore. That has to be some sort of evolutionary relic speaking to very different ancestral environments.
Polar bears instincts accommodate both treading water and swimming within moments:
Newborn babies can swim, obviously with no practice. In fact isn't it a classic example of an instinctive human behaviour?