In the near future there will be technology that allows people (aquanauts) to stay very deep underwater without the aqualungs or any other device. Aquanaut - a "man-cuttlefish", "man-fish", professional researcher of ocean depths. When working in deep water, he does without pulmonary respiration using special gills. Bony voids of its skeleton by under the influence of a special hormone ("Buridan hormone") are filled with fluid, so virtually incompressible, and the density of the body is close to the density of water.
Short novel "Akvanavti"