In the ocean, solar energy is rapidly absorbed or reflected in the upper surface, leaving little energy to penetrate deeper, and so deeper water is not heated (this also explains the lack of light deep in the ocean). Additionally, warm water is less dense than cold water, and so cold water will sink while warm water floats above it.
Thus, temperature of the ocean also varies from top to bottom, giving a vertical structure to most of the ocean. This vertical structure has a large impact on how life is distributed in the ocean.
