Looking at all the runs there doesn't seem to be much evidence of cavitation as yet, the spray pattern off the boards will give a good indication of when it is occuring. If you look at Finny's tracks on GPSSS he was maxing out at approx 2.4 knots over his 500m average speed. In Antoine's case if he did the same, considering they were on similiar sails and fins, with a 500 of 47.69 he would have seen very close to 50 knots, if not over on his GPS display.
I'd be interested to find out why you think cavitation wasn't occuring. Cavitation can occur at zero speed, simply by reducing the water pressure (which of course is silly).
Since the water is traveling over a curved surface, there will be some water which travels faster than the board speed (say 48knots) as it travels over the curved surface - in which case it is entirely possible for "localised 50knots" which could result in a localised cavitation. As Andrew mentioned, the guys don't look like they are running at optimum angle, so will not be able to unload the fin (thus not achieving zero AOA) and reduce the low-pressure.
For comparison -> there is a point in supersonic flight known as transonic flight, where airflow will travel faster and slower than Mach 1, depending on the plane's shape. It could be that fins are experiencing cavitation earlier than the raw-board-speed predicts.
