Fish and marine life are able to live because oxygen is soluble in water, including salt water/seawater.
oxygen \
Sodium chloride is very soluble in water.
Magnesium chloride is soluble in water.
Seawater is water (H2O which is a compound) containing soluble compounds but also insoluble materials. We can consider, with approximation, that the seawater is a homogeneous solution.
Hydrogen, oxygen, trace minerals.
oxygen \
magic.
There is more oxygen in seawater. Seawater contains around 5 to 10 milligrams of oxygen per liter, while the concentration of oxygen in the air is about 210 milligrams per liter. However, the total amount of oxygen in the oceans is much higher due to their vast volume.
Seawater and fresh water contains oxygen that fish absorb using their gill. So the substance is oxygen.
Oxygen enters seawater primarily through diffusion from the atmosphere at the ocean's surface. This process allows oxygen to dissolve in the surface waters, where it can be used by marine organisms for respiration. Ocean mixing and circulation also help distribute oxygen throughout different depths of the seawater column.
Two sources of oxygen in seawater at the surface include atmospheric exchange, where oxygen from the air dissolves into the water, and photosynthesis by marine plants and phytoplankton, which produce oxygen as a byproduct.
Oxygen enters seawater through diffusion from the atmosphere at the water's surface and through photosynthesis carried out by marine plants and phytoplankton.