Question

distinguish between the oxygen available to organisms living in the soil and oxygen available to organisms living in the water​

Answer 1

In water phytoplankton and macro algae produce Oxygen during photosynthesis. This is available to animals in the water, Zooplankton and Fish and also to the Aerobic Bacteria.
Oxygen dissolves in water, the saturation level is 9 mg / Liter at 20 deg Celcius. It is lower at higher temperatures. This dissolved oxygen is consumed by the fish, etc.

Oxygen is also directly added by photosynthesis carried on by aquatic plants, algae, and cyanobacteria. ...Animal and plants living in water get their oxygen intake by absorbing it fromwater. Oxygen is absorbed by diffusion through gills in animals and body surface in plants.

Answer 2

organism living in the soil have a more O2 than the organism living in water.

they don't have to breath faster.