when i did it - mind you it was a good 10 years ago - the vests that they made you wear were such that I would not have been able to go underwater if I wanted to. I did see a couple of people try to dive down and they made it just a couple of feet before they bobbed back up again (and were sumarily escorted out of the park, you have to stay on the surface, or at least they made you in the past). if they no longer have real flotation vests, there are underwater viewing windows where you will get much closer to the sharks than you will in the swim.
BTW all the sharks are bottom feeders and have no real interest in people.
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