Smooth Lightning doesn't works well underwater

Smooth Lightning doesn't works well underwater. It smooth the lightning a bit but we can still see some squares of shadows.

How to reproduce:

1. Do a big area of water without any block above it.
   
2. Enter it and stare right below you (you can also look at it out of the area):

• Without Smooth Lightning
  
• With Maximum or Minimum Smooth Lightning (note that these two do the same things)