I use both the 1D mark II N and the 50D and i'd probably recommend the 50D for surfing.
Like others have mentioned, the 15MP 1.6x sensor allows you crop more without using longer lenses, ultimately providing more detail with a given lens.
The colors and sharpness of the images from the Mark II N are nicer out of the camera, not forgetting the better high ISO performance.
I love my 1D a lot and use it everyday but I must say that the 50D is no slouch with regard to AF when coupled to any of my USM lenses! I have no complains whatsoever with the 9 point cross sensor AF system. It does what its supposed to do..FOCUS
Unless you're shooting in rainy weather which requires the weather-sealing capabilities of 1D, I'd recommend the 50D for your purpose.