at what depth does water stop looking darker from above?

[kathisterima]

I guess it depends on the source of light, like what time of the day it is, what latitude, how wavy the water is, how clean it is and maybe more. 

I think like 2m on the average beach you can't see from above, and it all looks the same, idk. 

But if what brocode said is true then the rocks or sand at the bottom shine back, so it must be somewhere inbetween 2 and 800.