[thedoc (OP)]

Oscar asked the Naked Scientists:
   What happens to light going down in water untill it gets dark in the deep sea?
What do you think?

[Colin2B]

Some is reflected at the surface, but mostly it is absorbed by plankton - including algae. There are also minerals in the water which absorb and scatter the light, and even water molecules will absorb energy.
There are some good articles by oceanographic institutes eg Woods Hole, of particular interest to divers if you want to look at the detail.

[guest39538]

Maybe something to do with pressure and density of the medium at the lower depths slowing the propagation speed of light down.

[Colin2B]

Maybe something to do with pressure and density of the medium at the lower depths slowing the propagation speed of light down.

Slowing the light down would merely delay the light getting there by a very, very small period of time.
The questioner asked a serious question, please do not disrespect him by offering an ill considered reply. Take the trouble to learn about light and its behaviour before offering an answer.

[Ethos_]

The questioner asked a serious question, please do not disrespect him by offering an ill considered reply. Take the trouble to learn about light and its behaviour before offering an answer.

I agree Colin.  Infection of the forum with biased views and polluting our discussions with this typical nonsense requires the application of some effective antibiotics. 

[chris]

The hydrogen bonds in water are strong absorbers of red light; this is why blood looks black underwater. As a result, not much light at the red end of the spectrum penetrates the deep ocean. Absorption of the red also makes the underwater world look bluer.