Because Most Energy is EXCHANGED at the surface!
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
The temperature in the marine biome varies depending on location and depth. Generally, surface water temperatures can range from below freezing in polar regions to over 90°F in tropical regions. Deeper ocean waters tend to be colder, with temperatures near freezing at the ocean floor.
Earth's crust is composed of the continents and the ocean floor.
sea floor spreading
basalt is the common rock
Subduction.
Pressure increases as you approach the ocean floor.