I had my D80 with only the 18-135 kit lens for a couple of months before I added a Nikkor 80-400 VR and also sometimes don't take the 40-800 say up a mountain or when I know I am going to get wet.
I have rummaged some taken at 135mm and found 1 instance of this phenomenon and it appears to be the addition of the speed/aperture combination that is the unusual one. Mine is at 1/800 f6.3. It is not just the blue sky as another at 1/250 f16 straight into pale blue sky shows no trace. PS found another at 1/200 f9 and added it to gallery below, so can't blame speed.
I have posted these in my Inbox
http://upload.pbase.com/edit_gallery/crisscross/inbox which I don't normally use and it may need password: D80
I was using f6.3 when I knew I wanted background out of focus. Normally for a seascape such as yours I would be using f16 or so at 1/200. The answer seems to be to use more normal settings! Or use slightly wider angle and either crop corners (as happened naturally in my examples) or heal brush out. Normally I always use the 80-400 down to 80 and only the 18-135 as a wide angle.
Generally the 18-135 is a superb lens. And only especially so taking weight and give-away price into account.