Why is the seawater salty?

Hint: Although seawater contains about 35 grammes of salt per litre on average, the oceans and seas are not uniformly salty; typically, the closer you get to the poles, the less saline the water becomes, as the concentration of salt is diluted by freshwater released from the ice of the frozen poles.

Complete solution:
The salt in seawater comes from rock, laden with elementary salts, including sodium, chlorine and potassium, which huge volcanoes from the depths of the earth have spewed out as magmatic material.

Carbon dioxide ($CO_2$) can form carbonic acid ($H_{2}CO_3$), a weak but corrosive acid when combined with water ($H_{2}O$). This carbonic acid rained down slowly breaking through and releasing the trapped salt into rainwater on salt-rich soil. Slowly, the runoff brought the salt to surrounding lakes and rivers, which carried it to the seas in turn. Although the volume deposited by any one outlet was tiny, the contribution of millions of outlets progressively increased the salinity of the oceans over millions of years. The process continues further.
A fair proportion of the salt released from the rock is used by living things along the way from rock to sea. Salt, controlling the amount of fluid in cells and neuron activity, is vital to both plant and animal life. The salt is released to begin its seaward journey when an organism dies and decomposes.

Note:
Acid rain isn't the only way that salt feeds the oceans. A significant role is still to be played by continuing volcanism. Hydrothermal vents allow seawater to return to the surface, which has seeped through the rock of the oceanic crust. The water is superheated from the magma below and dissolved minerals trapped in the crust as it travels up, erupting like mineral-rich steam.