What factors affect the temperature of seawater

Two of the most important characteristics of seawater are temperature and salinity - together they control its density, which is the major factor governing the vertical movement of ocean waters. The temperature of seawater is fixed at the sea surface by heat exchange with the atmosphere.

Temperature of seawater is affected by heat input from the sun.

Explanation:

More dense seawater sinks below less dense seawater and thus the oceans circulate partially based upon their temperature and salinity.