Why is sea water salty?

The salt comes off the land. Rain absorbs carbon dioxide from the air and becomes slightly acidic, and as it runs over rock it slowly erodes minerals and carries dissolved salts away with it.

River water carries salt too — just so little that you cannot taste it. What makes the difference is a property of the sea: water evaporates from it, and salt cannot. Over hundreds of millions of years rivers have kept delivering salt while evaporation kept removing water, and the salt concentrated.

Some salt also enters through volcanic vents on the sea floor. And the balance is fairly stable — some salt is taken back up by marine organisms and locked into sedimentary rock, so the sea does not simply keep getting saltier without limit.

The sea is not equally salty everywhere either. Where big rivers pour in fresh water the salinity drops, and where evaporation is high and rainfall low — the Red Sea, for instance — it climbs. The northern Bay of Bengal is comparatively fresh, because so much river water empties into it.