The ocean acts like a sunlight filter. The ocean is blue because water absorbs colors in the red part of the light spectrum. Like a filter, this leaves behind colors in the blue part of the light spectrum for us to see.

The deep, vibrant blue of the Pacific Ocean is a result of physics and the "optical properties" of water. When sunlight—which contains all the colors of the rainbow—enters the ocean, the water molecules and particles within them act as a filter. Water naturally absorbs light at the "red" end of the spectrum (long wavelengths) very quickly. As the light penetrates deeper, the oranges, yellows, and greens are also absorbed. Blue and violet light, which have the shortest wavelengths, are the last to be absorbed and can penetrate deepest into the water. This blue light is then scattered in all directions by the water molecules and reflected back to our eyes, making the ocean appear blue. The Pacific is particularly famous for its clarity and deep hue because it is so vast and deep; in its center, there are fewer "suspended sediments" like sand or silt from land, and lower concentrations of phytoplankton (which can turn water green) compared to coastal areas. This purity allows the blue light to scatter even more effectively, creating that iconic, bottomless "Pacific Blue" that has fascinated mariners and scientists for centuries.