At the "Big Pipe" contest the star gets trapped under water when her leash gets caught on some rocks. You see her undo the velcro on her ankle and then abandon the board (with the leash attached). About 20 seconds later when she is diving below the oncoming waves you can clearly see the leash re-attached to her ankle.
When the girls arrive at the beach at the beginning of the movie, they observe somebody who's getting worked by a HUGE wave. As he gets out of the water, they comment on him "he hit the reef" - "that's gotta hurt". This was not fake: The guy who did this REALLY got hurt that way and almost lost his eyeball.