Swimming under water is easy. In the "How To" website I googled, I was surprised to know that there is in fact a step by step way to do it. Here they are:
Step 1: Exhale as much of the air in your lungs as possible, and then inhale as much as possible.
Step 2: Repeat the first step one more time, and then slide beneath the surface of the water, holding your breath.
Step 3:Push off hard (provided there is something to push off of) and streamline your body, with your legs together and your arms extended above your head. Point your toes and try to push your shoulders up to your ears.
Step 4:Pull your knees up to your chest, kick out and apart, and then squeeze your legs together. This is the same as the breaststroke kick.
Step 5:Turn your palms outward and pull your arms all the way down to your sides. Bend your arms and cup your hands to push as much water back as you can with your pull. This motion is done at the same time as your kick.
Step 6:Glide as far as possible in your streamline position.