Actually, there are several answers to this question-- but I'll start by saying that your friend is basically right, the ocean is blue primarily because you see a reflection of the sky which is usually blue. (However, as you notice, at sunset, the ocean reflects the sunset colors of red, orange, ping, and yellow.) If you take your cup of water outside, what color does the top appear? (Remember that at the beach you can only see the top of the ocean.)(If you have a pair of swimming goggles, you can hold your breath and see the color of the ocean from under the surface. -- It may surprise you!).

A second answer is that a large volume of water is indeed very slightly blue-- but you won't see this effect in pure water unless you have a very large tank -- tens of feet long. Most of the color you see this way is due to water impurities -- even purified water has dissolved oxygen which can form very light blue complexes.

Finally, even small particles of dust (which aren't colored) in large volumes of "clear" fluids will have a bluish cast due to the physics of light scattering. (this is why the earth's sky appears blue). However, there is a lot of dust on Mars-- have you seen the pictures of the sky there taken recently? Be sure to check that the photo is "true color" but the color of the dust and its size can make a difference.

Water absorbs more of red light than it does blue.To actually see a significant difference with our eyes the light must travel about thirty feet in the water. Since the sunlight contains all colors, if you look at water more than about fifteen feet deep it looks blue. However a cup of water looks clear to us because it is so very slightly blue that our eyes cannot see it.

Now all this works if the light falling on the water contains all colors. If it doesn't as maybe in the evening or night, then the color of the water changes accordingly to green, red gray, black; just like a red cloth in a blue light looks black! So color also depends on what light you are seeing in!

So now, think about whether water has a "real" color...