Answer:
Fish get oxygen from water through their gills - they require oxygen to survive, so they have to be getting it from the water.
Step-by-step explanation:
The concentration of dissolved oxygen and carbon dioxide are very important for marine life forms. Although both oxygen and carbon dioxide are a gas when outside the water, they dissolve to a certain extent in liquid seawater. Dissolved oxygen is what animals with gills use for respiration (their gills extract the dissolved oxygen from the water flowing over the gill filaments).