Why is the ocean blue?

The ocean is blue mainly because water itself absorbs light selectively. As sunlight, which contains all colors, penetrates water, the water molecules absorb the longer wavelengths, reds, oranges, and yellows, first and most strongly, while letting the shorter blue wavelengths pass deeper and scatter back to your eye. So the deeper the water, the more red light has been stripped away and the bluer the remaining light appears. This is a genuine property of water, not an illusion: it means the ocean would look blue even under a gray sky, and it is why a swimming pool or even a deep enough glass of pure water takes on a faint blue tint. Deep, clear water can look a stunning deep blue precisely because so much non-blue light has been absorbed by the time light returns from below.

The common belief that the ocean is just reflecting the blue sky is only a small part of the truth. Sky reflection does contribute, especially to the surface color on a clear day, but it is secondary; the primary cause is water's own absorption and scattering of light. This also explains why the ocean is not always blue: its color shifts with what is in it and around it. Water rich in algae and plankton looks green (the chlorophyll reflects green light), sediment-heavy coastal or river-mouth water looks brown or murky, and shallow water over white sand can look turquoise because light bounces off the bright bottom before much red is absorbed. The famously vivid blues of tropical seas come from very clear, deep, sediment-free water. So the deepest, purest oceans are blue because of the fundamental physics of how water treats light, with the sky adding only a modest reflective sheen on top.