Whenever the player stands in water, with the light level being something noticeable (8 to 0 or so), the surrounding light level in the world, including the water, gets gradually brighter. It's as if the shading for being inside a block were reversed only for water. I tried teleporting myself into a block, where my head and torso would be free, but the shading was correct.

This also happens at night, but I only have an example for one at day: https://www.dropbox.com/s/wn42bcd037m7t00/Water%20Shading%20Reverse.mp4?dl=0

After reading 18w08b 40 minutes later: oh...well it shouldn't work above water anyway.