Ocean Currents

The ocean currents are caused by the gravitational pull of the moon.
There are three types of currents:
              Ocean Currents
              Waves
      And tides.

Most currents flow under the surface of the water and you can tell them apart from the water that they're going through because of temperature and salinity. 

(Sa-lin-i-ty, is a measure of the quantity of dissolved salts and other solids in a mixture such as sea water.) -From the science dictionary.