The number one factor for determining what living things can survive in a body of water is
temperature
density
depth
salinity

Salinity

Explanation:

Salinity can be natural in some of the natural biomes such as sea water and estuaries. There are some aquatic species which are capable of adapting and surviving in the change in concentration of salt. Those species which are incapable of adapting in the saline environment they decline in number, migrate to different location or even they may extinct. Organisms can tolerate in conditions like changing temperatures, density, and depth.

On the basis of the above description, salinity can be the number one factor for determining the fact that living things can be survive in a body of water.