How to breathe underwater?

How people breathe underwater is as follows:

People stand in the shallow water and practice the movements of upper limbs and head repeatedly. Open your mouth and inhale as soon as possible, but don't inhale into your lungs at first. Take a big breath in your mouth first and then inhale.

These advantages are to prevent the head from being held high enough, or the water surface is rough, the mouth is not air, but water, and the nose is prone to water. Breathing through the nose can effectively prevent water from entering the nasal cavity, so that the breathing time can be shortened when the head is raised, and the neck will not be tired and the swimming speed will be accelerated. Bowing your head and raising your head, breathing and inhaling can effectively prevent your nose from entering the water.