Distant sky visible while underwater
See screenshot.
While swimming underwater, if you are far enough from land that you can get an unobstructed view of the horizon, the edge of the water surface ends, making the skybox visible.
An easy fix for this might be swapping out the skybox while underwater with one that is a flat color the same as the extreme edges of the water surface.
Not a very serious bug, but a bug nonetheless.
(RM-3865 from redmine: created on 2017-05-12 by Phillip Lloyd, )