I have a 40D and use a 70-200 f/4.0 for all my outdoor work.
For windsurfing, even when shooting at Pelican Pt, it's not long enough. I'm looking at hiring some longer lenses in the near future to find out what length is going to be ideal.
I'm looking at testing the 100-400, which is a good action lens, or the 70-200 f/2.8, with a 2.0x extender ring.
One of my friends has the 70-300mm and she uses it rather well for nature work.
Those 18 megapixels mean that you might be able to do some fairly serious cropping, which is the poor man's zoom.
Some examples of the shots I've done in the past with my kit.
pelican pt -
http://www.flickr.com/photos/japester/sets/72157613989221723/mettams pool -
www.flickr.com/photos/japester/sets/72157623048187097/lancelin -
www.flickr.com/photos/japester/sets/72157612396390687/The Lancelin ones were from the judging platform, not the beach and even then, there is some fairly serious cropping on most of them. :(
Nothing in our price range is feasible for shooting the waves from the beach. You'd need something like a 1200mm lens to make that work. A boat will be cheaper.