Chris, you'll really want to read this artice:
Also, I have the 100-400L, and I use it to shoot at the beach all the time. Dust is a non-issue with this lens. I'm not saying it doesn't get a little dust in it, it does. But this dust cannot be seen in the images it produces. If you don't believe me, try this: Tear a tiny corner off a piece of paper. Wet the piece, and stick it on the front of a lens. Now look through the lens (attached to a camera, of course). You probably won't even be able to see the paper!The point is, dust on or in a lens is practically irrelavent.
Hope this helps.