Why does the sea water move?

Wind.

When it blows for a long time in the same direction, it causes a part of the sea water to move and forms a huge and invisible river called a current..

Sometimes, the sea moves by itself. When surface water is cold it sinks to the bottom and brings a warmer stream of water to the surface.

Sea water also moves because the earth rotates and, with her, turns all the water of the seas. The rotation of the Earth creates currents of water in the seas and oceans.