Fish can't get Oxygen directly from the water, as their bodies are covered in protective scales.
However, they have a very effective respiratory system to allow them to live well in water.
Fish have Gills, made up of lots of thin layers of tissue (This makes a large surface area) with a rich blood supply. They're thin, so there is only a small distance for gas to diffuse across, and the surfaces are always moist as they work in water.
In bony fish, water is pumped over their gills continuously to maintain a concentration gradient. These fish, like some shark, have to keep moving so water is constantly pumped across their gills, and they can breathe.
Gills don't work out of water, so a fish out of water will suffocate. Without water surrounding the gills, they stick together. There then isn't a big enough surface area available for fish to get the Oxygen it needs to survive.
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