the ocean is blue because there is blue light, from the sky, shining through it. if there was a different color light on the water the waters color would change accordingly. it has nothing to do with algae or lights ability to "penetrate" water, at least not on the scale youre talking about. take a colored light or laser pointer and shine it on a clear glass of water and the water will take on the color of the light.
look, a purple light in a pool makes purple water