The concentration of salt in seawater varies vastly in different seas
worldwide and even at different depths within the sea. Sea water contains an average of 4 % of dissolved salts (percentage by mass) of which :

Choride – 55%
Sodium – 30%
Sulphate – 7.5%
Magnesium – 3.7%
Calcium – 1.2%
Potassium – 1%
Bicarbonate – 0.3%
Bromide – 0.2%
Others – 1.1%

In fact, these may look familiar. Salt-water, is actually concentrated freshwater. When the rivers flow into the sea, the concentration becomes greater in seawater because of evaporation caused by the heat of the sun. The water evaporated and leaves the salt behind and then falls as rain again. If the rain falls over the land, the rain reaches out for more salts and the cycle start all over again. This phenomenon does not applies to sea alone. Any body where fluid loss through evaporation is greater than that through drainage by a river will become salty.
However, there are many complex and partly quantified forces at work here. Recent theoretical calculations to predict how salty the seas are have generally come up with answers, which conflict with actual experience.