Thank you minniev.
I also noticed the "wonkiness" in the colours around the edges when I processed the raw files in this burst of images.
Maybe I'm wrong but I put it down to most likely being caused by a too slow shutter speed.
At 1/800s the second curtain of the shutter is following close behind the first curtain across the sensor allowing only a thin slither of light onto the sensor during the shutter actuation. Since the subject is moving very fast in the scene perhaps there is some sort of "double exposure" effect happening because the thin slither opening in the shutter is moving too slow across the sensor.
Below is an example from an earlier shot in the burst (100% crop). There is a clear reddish cast around the keel of the surfboard.