Fishes use the oxygen from the water in order to survive.
Fishes are famous for their ability to breathe underwater, this can be attributed to their gills. The gills work in a similar way to lungs, which allow air/water to flow in and the lungs/gill extract the oxygen from the air/water, replacing the oxygen with carbon dioxide and removing it from the body.
Gills are tissues that are made of filaments. The filaments have small rows and columns of specialized cells that are grouped together, known as the epithelium. The filaments are responsible for the exchange of oxygen, carbon dioxide, acids and ammonia.
Most fishes have four gills on each side. The gills work when the fish takes in water through the mouth and closes the mouth, pushing the water to go through the gills. The gills absorb the oxygen from the water and replace it with carbon dioxide before pushing the water out. The fishes employ a countercurrent exchange system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other.
At one point in time, it is believed that every organism had gills in order to breathe underwater but with evolution and the shift from living under water to above land resulted in the higher vertebrates to lose this ability. On land, gills can result in the creature to suffocate.
This is due to the density of water and air. The density and movement of water result in the gills to be inflated, allowing the fish to constantly take in water and push it out to acquire oxygen. If the fish is brought out of the water, the pressure of the air results in the gills to collapse, causing the creature to suffocate. Hence, fishes require water in order to breathe and survive.