During Earth's early years, the oceans were only slightly salty. But when the first form of precipitation fell on the lands, the water flowed over the earth and naturally deposited minerals into the sea. As this process continues, the ocean has become saltier and saltier. An interesting fact about this, is that rivers and streams flowing through the United States alone deposit 225 million tons of dissolved solids, and 513 million tons of sedient into the sea! Throughout the world, rivers carry an estimated 4 billion tons of dissolved sodium (scientific lingo for salt) naturally into the sea!!!
Also, oceans become saltier and saltier each year, because:
a)As you all know, in the Hydrolic Cycle (water cycle), salt cannot be evaporated along with the ocean water. So in simple terms, water can evaporate, but salt cannot, so the salt has to stay behind and build up some more.
b)Salt also has an "anti-freeze" effect, there for, causing the ocean water to freeze near polar areas, and the salt again lingers in the sea, building up and up and up and up.