What makes the daytime sky look blue?

The sky appears blue because sunlight, though it looks white, contains a full spectrum of colors, and shorter wavelengths like blue scatter far more strongly off atmospheric gas molecules than longer wavelengths like red, a phenomenon called Rayleigh scattering. Our eyes are more sensitive to blue than violet, and violet gets absorbed higher up, so the net effect is a scattered blue glow arriving from every direction.

The ocean's blue is largely a separate effect from water absorbing red light and reflecting blue, not a major contributor to the sky's color, and the air itself has no inherent tint — it's transparent until sunlight interacts with its molecules.

This same scattering explains why sunsets turn red and orange: sunlight passing through far more atmosphere at a low angle loses most of its blue before reaching your eyes, leaving warmer wavelengths.