Best lens for surf photography?

I live in cornwall and do a lot of photography but I want to start doing some surf photography. I was wondering which is the best focal length for standing on a beach and taking the photos.
I have a nikon D5000

I use a Nikkor 200-400 mm f/4 and 600 mm f/4 lens when shooting surfers

If your budget will not support those two lenses, try the new AF-S 70-300 mm VR lens

I agree with fhotoace's recommendations. Perhaps I could add the suggestion of a the 300mm f4 as an middle of the range alternative, the Sigma 150-500mm is also an option if you wanted flexibility.

I should also add that shooting with the 200-400, 150-500 Sigma, 600mm f4 should be shot with support (probably a heavy duty monopod), the 600mm may be easier to use with a Manfrotto 393 on top of whatever support you're using. The 300mm f4 and 70-300mm lenses can be shot handheld, though having a support may allow you to do some more creative shots.

How far out is the break? Borrow or rent the longest lens you can find for your camera and give it a try. If the break is way out there, your surfers will still be little dots and you'll need a longer lens. It's hard to give you a number when we don't know how far away the subject is.

I would actually suggest getting a water proof case (make sure it goes to at least 50m) and getting in amongst it. You'll get better shots