According to NOAA, the ocean was originally not very salty but became saltier over time as rivers eroded the land and delivered the dissolved minerals to the ocean. At the same time, salts crystallize out of the water and are deposited on the ocean floor. This input and output are now more or less balanced so the ocean is not getting saltier. Apparently, this salt cycle involves about 4 billion tons of new dissolved salts being added to the ocean each year and about the same amount being deposited from the water to the ocean bottom.

So, why aren’t rivers salty? Apparently, it is because rivers carry only a small amount of salt and are kept fresh by constant rainfall, whereas the ocean has been accumulating salt for the last 4 billion years.

Lakes that don’t drain to the ocean, like the Dead Sea, can get salty over time, just like the ocean.