Just learning how to swim?
How to swim underwater. Push off the wall in a streamlined position, and begin to glide under the water about 6 to 8 inches under the water, and begin to do the breaststroke arms with the breaststroke kick, without coming up for a breath. This will help you stay underwater for a certain period of time without needing to come up for a breath.
If you would like to have a great swim underwater, it's suggested that you exhale a little bit so your body can stay underwater while you move your arms and your legs to push yourself through the water. The arms are kind of down like the butterfly.
You push all the way down with your arms to your hips, kick with your legs, and then recover your arms by bringing them as close as possible to your body, and reach forward to begin the repetition again. Push down, bring your arms down all the way to your hips, and come up with your arms forward. If you begin to run out of air, come up to the surface, take a breath, and then dive down again and repeat the butterfly arms with the breaststroke kick in that repetition. That's how you swim underwater.