Steps to reproduce:

• (I used flat worlds) make a hole to the bottom most layer
• place water at y=0 (it can be free, without support, I placed a redstone lamp next to the water and couldn't see it)
• get your head into the water, sothat the feet are at approx. -1
• try to see things, only moon and sun can be seen, sometimes, seems to depend on y height of the player
• I could see the contours of things infront of the sun

This is one bugged (pun not intended) me quite a bit in creative, obviously it won't affect survival very much, except maybe skyblock a bit. In a pool I could swim in a way to get my screen the same blackness without dying.