Water dissolves minerals easily. Salt is one mineral that dissolve very well in water. Now form where does the salt come ? Here are the various reasons why oceans have so much salt in its water-
From where does the ocean get water? From lakes and streams. From where do lakes and streams get water? From rain. Rain water has salt. This water mixes up with lakes and streams that ultimately join the oceans.
The water in lake and streams passes through soil, rocks and mountains. The soil, rocks and mountains have various minerals including salt. The following water keeps absorbing these minerals. This way oceans keep getting a supply of salt.
The amount of salt concentrated in an ocean also depends on how much evaporation takes place. For example-Dead sea has the higher level of salt.(salinity) because there is no outlet to the sea. No freshwater is being added to it or none leaves. The water keeps evaporating and the salts keeps depositing back.
They say if we collect all the salt from all the oceans, we can build a 40 storey building of salt!