I agree with the posts above. There is no single cause for the appearance of colours of the sea. The colours depend on many factors such as the brightness of the sky, the concentration of particles suspended in the water and the position of the observer.
The blue of the sea mainly due to scattering of light by molecules of water. Sunlight is made up of light waves of different wave lengths, each of which is seen as different colour. The shortest light wave appears blue while the longest, red.
When the white light is intercepted by particles of solid, gaseous or liquid matter, the various colours get scattered. If the intercepting particles are very tiny the shortest light waves get scattered the most.
When sunlight hits the sea, it is reflected by the molecules of sea water. The molecules being very small, they scatter the blue component of the light the most and so the sea appears blue.
Well technically eyesight-wise yes the sea is blue, but really the sea has no colour it is just transparent like water. The blue in the sea is the reflection of the sky. Have you realised on a cloudy day the sea is not as blue.