why the salty sea water?

Rain water, or precipitation, is slightly acidic due to dissolved compounds. When this acidic water beats down on the land and rocks, they end up dissolving the minerals in the rocks and return them to the water. The minerals are salt compounds-- not just sodium chloride. A salt is any ionic compound metal and a nonmetal, such as calcium chloride or magnesium chloride. The salts then flow down rivers and streams to their final destination in the oceans, where they remain. Over the course of millions of years, this salt has accumulated to be a substantial component of the oceans.