Why can not I breathe underwater like fish?

Have you ever thought why we can not breathe underwater like fish  instead of using oxygen cylinders and masks even though other fish are  using oxygen to breathe just like we also use oxygen?

The  main reason people can not breathe underwater is the difference in how  oxygen is delivered in humans. When a person breathes, air travels from  the nose to the windpipe and then into the lungs and the airways  gradually tighten in the lung. Eventually, alveolae, Through which the oxygen passes through the membranes of the lung into  the bloodstream and then takes out carbon dioxide from the blood and  then outside the body in the process called exhalation.

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As  for fish, they also need oxygen to survive, but they do not have a lung  designed to extract oxygen from the air. Instead, the water is passed  through its specialized organs, the gills that separate the oxygen from  the water to enter the body of the fish and expel the rest of the other  gases in the Water

But  there are some marine mammals that live in water and do not have gills  like whales and dolphins, and those marine organisms have a system to  hold oxygen inside their bodies for long periods of time under water but  ultimately need those marine organisms to rise to the surface of the  water to exhal and breathe oxygen again