Question

why the sea water is salty?

Answer 1

Ocean water is a complex solution of mineral salts and of decaying/decayed biological matter that results from the abundance of life in the seas. Most of the ocean's salts were derived from gradual processes such the breaking up of the cooled igneous rocks of the Earth's crust by weathering and erosion, the wearing down of mountains, and the dissolving action of rains and streams which transported their mineral washings to the sea. Some of the ocean's salts have been dissolved from rocks and sediments below its floor. Other sources of salts include the solid and gaseous materials that escaped from the Earth's crust through volcanic vents or that originated in the atmosphere. This process has been occurring for hundreds of millions of years, and therefore early oceans wouldn't have been as salty as they are today.
The ocean is not 'diluted' by the addition of fresh water through rain and rivers because the saltiness of the ocean is the result of several natural influences and processes, the salt load of the streams entering the ocean is just one of these factors. In addition, salts become concentrated in the sea because the sun's heat distills or vaporizes almost pure water from the surface of the sea and leaves the salts behind (this process is part of the continual exchange of water between the Earth and the atmosphere that is called the hydrologic cycle).
Because the ocean produces its own salt from the sun and sea sand!!! MLTL !!!

Answer 2

After years and years of river inflow and evaporation, the salt content of the lake water built up to the present levels. The same process made the seas salty. Rivers carry dissolved salts to the ocean. Water evaporates from the oceans to fall again as rain and to feed the rivers, but the salts remain in the ocean.