To answer the OP's question - In the busy season, boats often have a 2nd departure later in the day.
Noon or thereabouts. To take advantage of that, you need to deal directly with several of them as you get nearer to the date you would want to go. Get feedback about their bookings so you can tell whether it would be likely for them to have a 2nd sailing on the day (or days) that best fit your schedule.
I am here now (near Gansbaai) and I have been seeing a 2nd group arrive from time to time, so business is
good enough that some boats are indeed operating twice daily.
That's the answer. You cannot be sure, very far in advance, if there will be a 2nd sailing. And as Hank notes
above, it's possible to book your trip - and get a call the evening before (they will always ask for a contact number when you book) to get a call telling you the sailing has been canceled because they can see that the weather or seas won't be good enough to go out.
It's true that there are two shark cage operators in Simons Town - but their rates are quite a bit higher.
One of them, Apex Predators, is the boat where so many of the videos were filmed that show the sharks breaching and seizing seals as they rise from below and strike. But - Apex makes a point of saying that: A. They don't guarantee this behavior and B. They only expect it might happen during their morning sailings. This is relevant, because to get to their boat in time for that morning sailing from lodging in Cape Town, you still need to be leaving your hotel or B&B very, very early. I had clients in March who scheduled the pickup from their hotel in the city for 545AM.
Dave Patterson - Owner - The Africa Safari
Edited: 4:27 am, May 10, 2013