I'm relieved that Peter was able to provide an answer while I worked out mine:
The predominant source of heat on the Earth's surface is provided by the Sun. The Sun produces heat through the process of fusion, changing hydrogen to helium within the extreme heat and pressure that exists within its core. This generated heat migrates to the surface where it travels in the from of light radiated in all directions into space.
For the sunlight reaching Earth the properties of the surface determine what happens to the sunlight that reaches it. Some surfaces are more reflective than others redirecting the sunlight back through the atmosphere and out again into space. The sunlight that is absorbed releases the heat energy it carries into the absorbing material.
Sea water is somewhat transparent allowing some of the sunlight to penetrate below the surface before it is absorbed and releases its heat energy. Once absorbed the mixing of wind and currents also contribute to distributing and dispersing heat further below the surface. This distribution of heat away from the surface reduces the rate at which the surface of the water is heated.
As Peter pointed out, water has a very high
heat capacity
An equal volume of water absorbs more heat energy than almost any other material before experiencing the same degree of temperature change.
ocean heat budget
The dispersal of heat and the high heat capacity of water greatly reduce the rate of temperature change at its surface.