They have observed that approximately the same amount of salt that rivers and hydrothermal process put into the ocean water is constantly being removed from the water.
In other words, the salt gets use up.
For one thing, some of the salt is used by organism.
Clams, foe example. remove salts from the water to make thier shells.
In addition, if more water is removed from the oceans through evaporation than is replaced by river water flowing in, salt starts to settle on the ocean floor.
Therefore, the level of salt in ocean water remains fairly constant.