The ocean is fed by rain, snowmelt and rivers, yet its water contains enough dissolved salt to be immediately noticeable. The explanation is not a single event but a geological process operating over immense periods of time.
Rainwater is slightly acidic and slowly weathers rocks on land. That releases dissolved ions which are carried into streams and rivers and eventually reach the ocean. Rivers therefore are not completely free of salts; they simply contain far lower concentrations than seawater.
NOAA identifies runoff from land and geological openings in the seafloor as two major sources of ocean salts. At hydrothermal vents, seawater enters cracks in the ocean floor, is heated and chemically altered by hot rock, then returns to the sea carrying different dissolved materials. Undersea volcanic activity can also contribute minerals.
Evaporation helps explain why the sea remains much saltier than rivers. Sunlight removes water from the ocean surface as water vapour, but most salts remain behind. Fresh water later returns through precipitation, continues weathering the land and transports more dissolved material back toward the sea.
The average salinity of seawater is about 35 parts per thousand, or roughly 3.5 percent by weight. Sodium and chloride are the two most abundant dissolved ions; NOAA says together they account for about 85 percent of the dissolved ions in seawater.
Salinity varies from place to place. Heavy evaporation can raise it, while rainfall, river discharge and melting ice can lower it. At the same time, salts are also removed from seawater through biological processes, chemical reactions and burial in sediments, helping create a long-term balance.
Ocean salinity matters for more than taste. Together with temperature, it affects seawater density and therefore contributes to large-scale ocean circulation. In that sense, the minerals washed from rocks over geological time are also part of the machinery that helps move water and heat around the planet.
Source: NOAA Ocean Service — Why is the ocean salty? and Why is the ocean salty, but rivers flowing into it are not?