Well, I am glad to be learning this about DX/FX - interestingly, the camera shop fellows told me that DX actually magnified the image, though of course at 10.5 MP. The idea conveyed was that I could shoot in DX and get more "reach" from my lenses - eg. a 300mm lens becomes a 450mm lens effectively.
I now hear that this is incorrect. Important news for me. I am not able to say why anyone would shoot an FX lens in DX mode at this point.
Back to the surfers: I was shooting in aperture mode, with an eye towards keeping shutter speeds above 1/1000, using various ISO settings to do so, using VR sitting crosslegged on the ground bracing as best I could & using breath control to try to reduce camera shake with 20 mph gusty winds, and shooting from far away. I think that in DX mode my matrix meter would be metering less expansive of an area than in FX. It was a mottled light day, with scattered clouds moving fast. I suspect that in DX the matrix was getting what on FX I would have metered with center weight metering. So, the camera may have picked different shutter speeds in DX than FX. Hence, images may have been with higher shutter speed in DX, and look more clear and sharp than some of the FX images. Add to that, I was shooting at slightly less than 300mm in DX mode.
I'll probably use spot or center metering and FX for such shots from now on, and maybe shutter priority & auto ISO. I now own a "travel tripod", so will hope to be able to shoot without VR & from stable position (though wind is a factor).
I do not want to beat a dead horse here, so thanks to all who responded, as I have learned some things which are important to me.