how deep before it gets dark?

[pipedope]

In a Florida retention pond it is completely black in less than 6 inches of water.

 

[Thalassamania]

When I was working with Sylvia Earle off Lee Stocking Key in the Bahamas, we brought up some green plants from, I seem to remember, about 900 feet. That means that enough light is getting through there to support photosynthesis at that depth.

 

[VinceDS]

From reading this thread, think it just depends on where you dive.

I have been to 160+ on the Hydro Atlantic in SE Fla. on days when vis is 70+ and, other then penetration, never needed to use my light...