Oceans and seas appear blue because they reflect the blue colour of the sky. I can prove this because if you looked at the sea on a cloudy, grey day, the ocean will also look grey.

Did you also know that there are some seas that are weirdly coloured such as the Red Sea which sometimes looks red because of red seaweed called algae that lives in it? The Black Sea looks almost black because it has a lot of hydrogen sulfide, which is a chemical that looks black.