Because Most Energy is EXCHANGED at the surface!
The sun's rays are absorbed by the water, warming it, and the water can also be warmed by the air at the surface. But as you go deeper, below a few dozen meters, the sparse sunlight that does penetrate has little effect on the water temperature.
The heating of the oceans, and shallow ocean floors, causes convection currents, as warmer water rises and colder water sinks. This creates substantial mixing in relatively shallow water. Both convection and wave action can mix water to several meters in depth, but below that the water can remain cold and dense, with less mixing and less oxygenation.
The surface of the ocean has a greater exposure to sunlight, wind, rain while the ocean floor is not exposed to such variable influences.
Sunlight in, radiation and convection out.
In order for temperature to change the involved heat has to have some place to go. The water at the bottom of the ocean is surrounded by ground and other water that are already at the same temperature. Where can extra heat go to? At the surface the air transports heat up and away so ocean heat can leak out (or in) to the water.
Lava
The opposite of bottom would be the top, so the oceans surface is the opposite of the ocean floor/bottom.
What do you mean with "floor"? The center of the Sun is around 20 million K, the visible surface around 6000 K.
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basalt
Earth's crust is composed of the continents and the ocean floor.
sea floor spreading
The Crust - The crust is Earth's solid and rocky outer layer, including both the land surface and the ocean floor.
floor
basalt is the common rock
Subduction.