Because the ocean reflects off of the sky so technically the sky is white but sense the sun causes the ocean to reflect onto the sky, the sky appears to be blue.
Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time. Closer to the horizon, the sky fades to a lighter blue or white.
