It occurs pretty much every year, (at least 3 of the past 4. years that we've owned there). Since the beach erosion that occurred a few years ago, the sharks will chase bait-fish pretty close to shore.
The rule of common sense has to apply here...don't go in the water when they're migrating. You rarely see one or two, they are in pretty large schools and a lot of helicopter activity is a good tip off.
There were a couple of pretty severe bites about two years ago, but they were well north of Singer Island.
I've never let them ruin my stay...and enjoy seeing something we don't get to see all that often.
For the short period that it happens...I'll stay out the ocean as long as they'll stay out of our pools.
Mike