The sea is blue because water, in large quantities, is blue. Also, when the light from the sun hits the water, it gets filtered so that redness is absorbed, allowing more of the blue color to be reflected.
Another reason why the sea appears blue is because it reflects the blue color of the sky. Little ocean particles serve as reflective devices, and because the sky surrounds the ocean, the water appears blue. The presence of plant life and algae can cause the sea to appear green, and it may even appear gray when the sky is cloudy. Water that contains large amounts of sediment may appear brown in color.