Fish require oxygen for survival just like other animals, but they filter it from water using specialized organs called gills. A fish draws in oxygen-rich water and passes it through its gills. The organs absorb the oxygen from the water, carrying it into the fish's bloodstream for use by its organs.
Gills are made up of many small filaments that contain capillaries, which give the organs a large amount of surface area. This is necessary due to the lower oxygen content of water compared with atmospheric air. Any given volume of air contains more than 25 times as much oxygen as the same volume of freshwater. As a fish swims, it creates a constant current of water over its gills, allowing it to extract as much oxygen as possible. Some fish have the ability to create this pressurized stream of water using their internal organs, while others must remain constantly in motion to draw enough water through their gills to survive.
Not every species of fish relies on gills for oxygen. Some fish have rudimentary lung structures or absorb oxygen through their skin. These must occasionally surface to partake of the oxygen-rich air instead of filtering it from the water around them.