The water moves from an area of high concentration of water (inside the fish) to an area of low concentration of water (high salinity; saltwater environment).

A freshwater fish placed in saltwater would die from water loss.

The water moves from an area of high concentration of water (low salinity; freshwater environment) to an area of low concentration of water (inside the fish).

A saltwater fish placed in fresh water would absorb too much water, swell and die.