why does the ocean stay full of salt water?

Answer 1

The water in the sea/ocean is salty.This is because most of the rivers and streams empty themselves into it.The rivers are strewn with rock and plant debris.
The rocks contain many minerals and salts(especially sodium chloride) which dissolve in the water.As the rivers drain into the sea, a considerable amount of salt gets dissolved in the sea water.
When these waters evaporate due to the heat of the sun, they leave the salt behind.The evaporated water is brought back to the land in the form of rain and the rivers carry the water again to the sea.
As this cycle is repeated, the amount of salt increases in the sea but the water content does not increase.May be,the seas too would have lost their salt if they had had oulets to let out the salt.

Answer 2

The salt comes dissolved in the fresh water that runs into it from rivers. As water evaporates in the regular evaporation/precirptation process, the salt stays behind in the ocean and becoames concetrated their (salt does not evaporate with the water). The Evaporated ocean water falls as rain, goes into the river system (or glaciers, etc.) and picks up more salt as it flows back to the sea, and the cycle continues.