Example: If one SCUBA diver looks past another SCUBA diver horizontally into negative space, how far away is the most distant emission of 'blue' light waves? Consider the sight angle in question to be that which offers the maximum perceived distance, which I speculate is horizontal or along the Mariana Trench to nullify the Earth's curvature or something else beyond my assumption.
Please disregard local water visibility conditions as in how far away an object can be discerned due to suspended matter. I am curious about the actual 'blue' region you see in an undersea photo. I assume this is scattered light from distant water molecules.
I suppose that, unlike the black of outer space between star light emission, the 'blue' of water is not the absence of emitted light from a source but a non-point diffusion of light.