When there is no light, as opposed to something like light from the sky at night, you can see the ocean floor much better.

Steps to reproduce:
1) FInd an ocean
2) Make a big solid platform of like 50x50 somewhere high above it
3) (Optional) Set the time to night
4) Look down from high up in the sky and compare the darkened water with the lit up water (which has sky access)
5) Notice you can see the ocean floor much better where there is not actually any light.