Nikon or Canon both have competitive models that start at around $700. Both have a great selection of lens to work with their low end DSLR's. I happen to prefer Nikon because I already have a number of good Nikon/Nikkor lenses. If you don't have any lens yet, either company will be a fine choice.
I went to a pretty good photo school in California for like 6 months and was told that for shooting action that Canon makes a better chip. The reason for this is that the way the chip is link with the image processors it allows for higher ISO without as much noise so when shooting in lower light it is easier to use a faster shutter speed but avoid the grain the i know i get on my nikon. However the lens argument is pretty good to, you can ultimately take great surf pics with the nikon or the canon so if you have lenses for either i recommend purchasing according to lenses.
Ditto on the recommendations you've had so far, especially on the lens purchases. Once you buy into a system, you sort of have to stay with it or lay out a much larger bundle of money all over again. Name brand lenses like Nikon and Canon are not interchangable, although you can get aftermarket lenses that will fit, their optics and engineering are not generally as high quality as the Nikons and Canons.
a used canon 20d will only run you about $300 for the body, and its still a great camera, its just been replaced 3 times so they go for cheap.
a canon 50mm f/1.8 brand new is $80
then a canon 70-200 f/4L goes for about $500 used i think, which isn't a bad lens to start out with for surf.
or for about the same price you could get the 200mm f/2.8 fixed lens