Everybody has different stories and mythological beliefs that why ocean is salty but scientific reason behind it is that some carbon dioxide is dissolved in the rain from the surroundings and water becomes slightly acidic because carbonic acid is formed with carbon dioxide and water. This acidic water causes erosion on the rocks and carries them in the form of ions. This is further carried to the streams then to rivers and finally to the sea most of the ions absorbed by water organisms and the left ones increase the concentration of salts with the time.
Millions of years ago primeval seas were slightly salty. Continuous breaking of rocks by the rain water and its transfer to sea for many years might have made the ocean salty. Rivers and streams of U.S alone carries 225 million tons of dissolved solids and 513 million tones of sediment approximately, to the ocean in a year.
Only rainwater is not responsible for the dissolved salts of sea other sources are sediment and rocks below its floor. It gets eroded by the movement of sea waves and also get stirred. Salts are also made up of the gaseous or solid material escaped from the volcanoes of earth’s crust or that are present in atmosphere.
When evaporation takes place, sun’s heat distills water, and pure form of it evaporates leaving behind the salts to the sea which increases the concentration of salts which shows that only long accumulation of salts are not responsible for its salinity. Seas in the polar areas are less salty because melted ice dilutes the concentration of salts in the sea.
Some lakes are landlocked so they taste same as the sea. They do not go further into river or sea. When water is evaporated from the lake it is left only with the salts and no water.
If the salts of ocean is evenly spread on the earth a layer of 500 feet will be formed.