<<The gunsight is slightly shaded/tinted I think, or polarized maybe (I know some aircraft in WWII had polarized gunsights) - so the sky can look darker through them, and reflections are cut from the shine on the sea.>>
Maybe, but the effect flickers on and off, which says to me it's a bug. It's also interesting that clouds disappear under the gun-sight.
http://i984.photobucket.com/albums/a...5-01-51-00.jpg
http://i984.photobucket.com/albums/a...5-01-52-00.jpg
http://i984.photobucket.com/albums/a...5-01-53-00.jpg
http://i984.photobucket.com/albums/a...5-01-54-01.jpg