why is the sea blue?

Answer 1

The answer above is correct, but needs more explanation.
The colour of the surface of the sea is a result of reflection from the sky, but the colour of seawater in bulk is blue for the same reason that the sky appears blue: The water molecules are much smaller that the wavelength of visible light; but the shorter wavelengths of the blue part of the spectrum react much more strongly with the water molecules than the longer wavelengths (the red and yellow part of the light spectrum). When the blue wavelength reacts with the water molecules, the light energy is absorbed, which makes the water molecules vibrate and re-emit the energy, at the same wavelength. Because the molecules vibrate in all diretions, the blue light is emitted in all directions (called 'scattering'), so when we loook at the water, it seems blue.

This effect can be enhanced if there are other particles in the water (eg, finely divided clay particles). That is why the water in a flooded abandoned quarry, for example can be intensely blue.
The effect is also enhanced if the water is solid, and you will notice a beautifully intense blue colour if you look down a crevasse in a glacier, or poke a hole in the snow with a ski pole.

Answer 2

There is no single cause for the appearance of colours of the sea. The colours depend on the brightness of the sky, the concentration of particles suspended in the water, and the position of the observer.

The colour of seawater is largely produced by the absorption and reflection of light of various wavelengths—by the water itself and by matter suspended in the water. Central to this concept is the fact that white light is actually the combination of wavelengths of light from the entire visible light spectrum. Each colour of the rainbow occupies a different wavelength of light, with red and orange being the longest and blue, green, and violet being the shortest.

When sunlight hits seawater, some of the colour components of white light are absorbed, while other colours collide with water molecules and are scattered. When a colour is scattered, it is reflected in all directions—including up to the surface and this can make the water appear blue...
Blue light is more easily bent, or refracted, than red light; thus, light refracted back from the surface of the sea appears blue. Furthermore, when you are underwater the water around you appears blue because more blue light is scattered back to your eye than red light.

Answer 3

A common misconception is that the oceans are blue primarily because the sky is blue. In fact, water has a very slight blue color that can only be seen in large volumes. While the sky's reflection does contribute to the blue appearance of the surface, it is not the primary cause. The primary cause is the absorption by the water molecules' nuclei of red photons from the incoming light, the only known example of color in nature resulting from vibrational, rather than electronic, dynamics.

Answer 4

Actually, it is not blue. Blue is the color you see when light reflects off of it's surface.

Answer 5

Because God made it that way.....lol..