An aquatic system lacking dissolved oxygen (0% saturation) is termed anaerobic, reducing, or anoxic.
In water, oxygen levels are approximately 7 ppm or 0.0007% in good quality water, but fluctuate.
Many organisms require hypoxic conditions.  Oxygen is poisonous to anaerobic bacteria for example.
Oxygen depletion  is typically expressed as a percentage of the oxygen that would dissolve in the water at the prevailing temperature and salinity. A system with low concentration—in the range between 1 and 30% saturation—is called hypoxic or dysoxic. Most fish cannot live below 30% saturation since they rely on oxygen to derive energy from their nutrients. Hypoxia leads to impaired reproduction of remaining fish via endocrine disruption. A "healthy" aquatic environment should seldom experience less than 80% saturation. The exaerobic zone is found at the boundary of anoxic and hypoxic zones.
Hypoxia can occur throughout the water column and also at high altitudes as well as near sediments on the bottom. It usually extends throughout 20-50% of the water column, but depends on the water depth and location of pycnoclines (rapid changes in water density with depth). It can occur in 10-80% of the water column. For example, in a 10-meter water column, it can reach up to 2 meters below the surface. In a 20-meter water column, it can extend up to 8 meters below the surface.