This is an important skill every parent must demonstrate starting to teach their child to swim at 2-18 months! There are specific cues to use and visuals your child will see. Use your bathtub and the time you bathe your child but before you soap them up.
Do not tell your child to close their mouth. We have a built-in reflex to close our epiglottis. Your kid swam in a womb for nine months. Just as a dog needs no instruction your child is made to swim naturally. But you can mess them up by telling them to close their mouth. That changes their focus from breath holding to closing the mouth and guess what happens. They will inhale and choke.
When you close your mouth you also lose the air lock in your nose. Water will go up into your nose 1 mm. to tickle those hairs, and they subconsciously send a signal to your brain to lift the head up. When your head goes up your hips go down and you cannot float level like an air mattress to be efficient.
Watch the video for some suggestions. And if you know parents with infants share this video with them to get their child starting to swim at home without a pool. This saves more lives.