Underwater use of scuba breathing is definitely not a "normal" thing! Normal definition of "normal breathing" is: on the ground, the spontaneous use of the nose to do suction, exhale movements, under the water ~ diver is to use the mouth to complete the work of the nose. (Full-face masks are not discussed here.) So if you want to "get used to" breathing underwater, breathing in underwater skills is a compulsory course!
On the ground, breathing action is almost ---- spontaneous! The brain stimulates the stretching of the chest muscles, the diaphragm drops, and the lungs expand to introduce air. When the muscles are relaxed, the gas is naturally discharged! Interestingly, it is not the body's need for oxygen (a decrease in oxygen partial pressure) that stimulates the brain to breathe. Instead, it raises the carbon dioxide level to stimulate the brain's carbon dioxide receiver, which in turn determines how fast the respiration rate is ! Oxygen partial pressure is also the second to affect the parameters.