You can't just add more in analogy with the way air has a percentage of oxygen. It holds as much as it does due to the laws of physics, and the air above will supply as much as the water can take up.
You would have to change the properties of water and/or oxygen so it desolves more. Or lower the temperature.
This graph is found here which also covers What Affects Oxygen Solubility? generally. This is a great article to go through.
That's how you can observe the answer: look at the sealife in arctic vs warm water. The abundance of life in colder water is due to increased oxygen levels there.
Another idea is to increase the oxygen at depths where it is currently not saturated. This entails more mixing. This normally happens with wave action over reefs, currents that sink oxygen-rich surface water, and storms.