I have just returned from visiting the future! Upon staying there for a while I did, of course, have to stay somewhere. So I stayed at the Underwater Hotel.
In order to breathe in these places filled with water, you must wear a heating pad which velcros around your middle. Inside the heating pad are lots of light-weight crystals which scientists created to absorb oxygen. The crystals absorb the oxygen in the water around you. The heating pad warms the oxygen. On one side of the heating pad is a tube injected into you. The tube takes the heated oxygen and inserts this into your bloodstream, as well as lungs.
Since your body needs 550 liters of oxygen a day, and the crystals cannot hold enough oxygen to give that much, then three times a day you must exchange these crystals for new ones, more filled with oxygen. You put the old ones down a tunnel through your hotel room wall. The employees working down below take the un-oxygen-filled crystals and bring them up to land to absorb more oxygen to be reused later.
Scientists have already invented crystals which hold oxygen. So if they could discover a way to make them hold more oxygen for a longer time, without having to weigh more, so you wouldn’t have to carry heavy objects around your waist all the time, the future itself would be a better place!