What makes the ocean blue even when the water is clear?

 Most people think that the ocean is blue because it is reflecting the blue sky. But this is not true.

The ocean is blue because of the way it absorbs sunlight.

When sunlight hits the ocean, the water strongly absorbs red, violet, and ultraviolet lights. The remaining light that we see is made up of the colour blue. 

However, the ocean may become other colours, including red and green, if light bounces off objects floating near the surface of the water, like algae.

Just how blue the water is depends on how much of it there is to absorb the light. Like if you had water in a glass, it would be clear because there aren't enough water molecules to really absorb the light.

This fact is clear if you look at unedited underwater photos that weren't taken with a camera flash or another fake light source - even the most vibrant of tropical fish look blue.