The sky is blue because sunlight is made up of many different colours of light but the blue light is the one that gets scattered across the sky the most and so we see the sky as blue. The sea is blue due to the fact that it absorbs most colours of light such as red but instead reflects blue light and so it appears blue. I think 🙂
Sunlight contains a mixture of colours visible to us, of which blue (actually violet) is the one that is scattered most by the molecules in the atmosphere, thus dominating our colour perception of the sky (our eyes have receptors specialized for blue but not violet). And if you look at something blue through a mirror (like the surface of the sea), it looks blue too.
The short answer is Rayleigh scattering. So what is Rayleigh scattering? I’ll try and explain…
The light that comes from the sun is made up of all wavelengths of light, which means every colour. We call this white light as all colours combined in light make it white. When the light from the Sun hits tiny particles in the atmosphere, even down to the individual atoms it is ‘scattered’ which is basically what it sounds like. Atoms refract and deflect the light and ‘scatter’ the light in all directions. Some light is bent more than others in this scattering, in fact the violet end of the spectrum is bent best. Think of a rainbow, the violet end is bent more than the red end. So, when you look at any are of the sky that isn’t the Sun, most of the light is made up of the light that’s been bent the most, this is because there is a much larger area of sky it can be ‘deflected’ from. All of this means that normally, the sky looks violet. Unfortunately, our eyes aren’t very good at seeing violet so the sky ends up looking blue.
The same process is behind the red sky at sunset. Because the sunlight has to travel further through our atmosphere the light that is scattered the most is ‘diluted’ more and so only red, at the other end is left.
As for the sea… it’s the same, only the light is deflected off the surface, rather than scattered in the atmosphere.