Sky is blue in color though it is filled of different colorless gases. Sky should be colorless but this is not the case. One and only the reason behind this blue color of the sky is that, when light rays coming from Sun inters the atmosphere of the Earth, it disperses in the atmosphere into different colors. The blue color has lowest wavelength and therefore has highest energy to remain longer in the atmosphere and therefore we see Sky as blue. While other color components have lower energy and get dispersed and not reach to our eyes.
Similar reason is there behind the blue color of ocean. to see the color of ocean the light ray falling on the ocean must be returned to our eyes after striking the bottom of ocean which is not possible. Therefore light interring the ocean never comes out and get dispersed in the ocean due to different molecules including water minerals salts its and as said above blue color has highest energy and therefore becomes dominant to other colors and thus color of ocean appears to be blue.
I therefore thing that blue color is the symbol of deepness, weather Ocean or some kind of river or lake, which appear to be blue in color when they are much more deep.