It is difficult to explain and understand with a short answer but it has nothing to do with the oceans which actually aren't all that blue anyway. The sky is black at night. How about that puzzling fact? When the light goes out the ceiling (sky) turns black. The Sun of course lights up our atmosphere and sends light toward us which appears white when viewed from a very high flying plane or from space. Actually the Sun's light is composed of all the colors, but blue because of it's particular wave length scatters more than the other colors so much so that we can't see the others as the air in our atmosphere above scatters the light. So the sky is blue and I am out of breath. So you might ask why the sky looks red at sunset and sunrise. No the ocean is not burning.
Sky isn't always blue. Have you ever seen a sunset? Sky is always red then. This is because during the day most of the shorter wavelength light (blue) scatters due to obstructions in the atmosphere . Hence we see it blue. During sunset the light has to travel greater distance through atmosphere due to the angle it hits the earth. Hence even though blue is scattered it's far of from us and hence only the red and yellow that pass through . Hence we see it red
The atmosphere is the mixture of gas molecules and other materials surrounding the earth. It is made mostly of the gases nitrogen (78%), and oxygen (21%). Argon gas and water (in the form of vapor, droplets and ice crystals) are the next most common things. There are also small amounts of other gases, plus many small solid particles, like dust, soot and ashes, pollen, and salt from the oceans.