The answer to this question lies in the mechanism involving three components: the light, an object (ocean) and human eyes.
Perception of Colors by Eyes:
The human eyes see an object when the light reflected by the object reaches eyes. The eyes can perceive only the visible spectrum of light in the wavelength range of 390 nm to 700 nm. The seven colors in the visible spectrum of light are red, orange, yellow, green, cyan, blue and violet. Each of these colors has a definite range of wavelength.
When light falls on an object it involves a process of absorption of colors of certain wavelengths and reflection of colors of the remaining wavelengths. To eyes, the appearance of colors depends upon the part (s) of the light spectrum absorbed by the object and the part (s) of the light spectrum reflected by the object.
Based on the spectrum of a reflected color, an object will appear of that particular color. For example, an object which reflects only red color of the spectrum and absorbs all other wavelengths will appear red. Similarly, an object reflecting red and green will appear red and green. An object that reflects all colors will appear white. Absorption of all wavelengths and reflecting none will make the object appear black. The eyes perceive a color when a wavelength range specific to that color interacts with the light receptors or cone cells in retina. Perception of different colors is based on the varying spectral sensitivities of different receptor types or cone cells.
The oceans appear blue because the blue color is reflected to eyes. But the various reasons that oceans appear blue are not all that simple as in the case of any other object appearing blue. The reasons oceans appear blue are explained below.
Mechanism of Blue the Blue Appearance of Ocean
A small quantity of pure water is colorless. When water is present in a larger quantity, it appears blue due to the pure water’s intrinsic property. For the same reason, oceans having huge quantity of water in the center and no impurities or particulate matter appear dark blue. When sunlight hits the surface of the ocean, small fraction of light is reflected back.
Most of light hitting the ocean surface penetrates it and travels down to some depth and interacts with the water molecules it comes in contact with. The longer wavelengths of the visible red spectrum (red, orange, yellow and green) are readily absorbed by the ocean water but shorter wavelengths (blue and violet) travel further down the ocean. In the ocean, very little light reaches deeper than 200 meters and no light penetrates beyond 2000 meters. There is no reflection of any color spectrum from the ocean floor.
When light falls on a substance, the observable color of the substance is an outcome of the interaction between the visible light photon and the electrons present in the molecules of the substance. In case of water, the absorbed light photons promote a state of highly excited vibrations in the nuclear motions of water molecule. These excitements lead to emission of intrinsic blue color of water.
There is also some scattering of light after colliding with any particulate material present in ocean, but this is very less in the deep ocean. The color of ocean in areas near coasts may be quite different due to the presence of particulate matter or phytoplankton. Chlorophyll pigment present in phytoplankton absorbs red and blue portions of light spectrum but reflects green light giving the ocean a blue-green to green color.
The appearance of ocean as deep blue is an outcome of complex interplay of factors such as clear sky and bright sunshine, reflection of sunlight from the surface of ocean, scattering of light by particles present in ocean, absorption of longer wavelengths and scattering of shorter wavelengths (blue and violet) by the ocean waves and a state of excited vibrations in water molecule. Due to scattering of blue light divers feel a deep navy blue color on the deep ocean.