The oceans are actually blue because of what's called a "scattering effect". Basically, the shorter waves that we see as blue have to bounce off more molecules of water before they reach our eye. They scatter many times before they get to us, and eventually they come back a deeper blue. This is where I'm going to start to get a little more technical. Water does absorb some of the scattered light from the sun, so as we get deeper into the ocean, less light reaches us.
