Sunlight penetrates the topmost ocean surface, below that it is always dark.
The sunlit top ocean layer is called the photic zone and its depth ranges from a few to hundreds of meters. Ocean color is linked to the optical properties of water and how light is absorbed and scattered by dissolved material and suspended particles, including phytoplankton cells. In clear tropical oceans, sunlight penetration is very deep and waters appear indigo blue in color primarily because of the optical properties of water itself. Ocean color satellites are optimized to detect the range of colors reflected from the ocean surface. Data from these sensors are used to monitor the concentration of chlorophyll an indication of phytoplankton concentration in the upper ocean.