This might be a lighting and fog bug, you can barely see underwater below Y:0

This is actually a lighting bug, and light propagation is being rewritten (most of the issues can't really be fixed in the current code). I will make a separate issue for lighting.

This actually is another lighting bug, because it also is incorrect serverside. One thing I should do is make the client fix it's own lighting bugs as player gets closer. This is what partial issue looks like:

It seems like lighting isn't properly updated in this case. I think I know the exact cause of this. Is it better after re-entering the world? If so, it's actually a duplicate of #6