In fact, different oceans can appear to be different colors. At a distance, oceans may look blue; but, waves on a beach may appear greenish.
Cold waters may appear a transparent green. Oceans may look gray, especially when stormy, or brownish near the shore if they are churning up sediment.
Objects absorb light from the Sun, which carries all the colors of the spectrum. They reflect back only certain colors depending on their physical makeup.
A ripe tomato reflects red; a lemon reflects yellow. The waters of the ocean generally reflect blue.