Question

why the sea water is salty

Answer 1

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.

Answer 2

Ocean water is salty because it contains large amounts of dissolved minerals. These minerals are often called “salts." Depending on which part of the world you are splashing around in, about 3.5% of the oceans' waters are made up of salts. Water near the equator tends to be saltier, while northern waters tend to be fresher, meaning they contain fewer salts.