SimonV and I have done Sharks Deep Dive and would HIGHLY recommend it. You start with an educational video, then move into a dressing room (men and women separate) to suit up for your 'dive'. When you reach the Shark Tank you have a few minutes to get used to the cool water (ok, cold) before you enter the diving cage.
Once inside there are handles to hold on to about half way down the cage so that you can stay submerged (but you can surface, as needed). The cage works its way across the tank and back, during which the view of the sharks and other sea life in the tank is absolutely incredible. A smaller fish joined us in the cage for a while, but the sharks can't enter (obviously). They do swim fairly close though, hence the warning not to put your fingers outside the cage.
You can also see other guests as they walk through the tunnel in the center of the tank. Many of them took our pictures, nearly everyone waved at us. And, if you're really paying attention, you can see diners inside Sharks Underwater Grill. We had lunch there later and were delighted to discover we could see the shark cage as the next tour went by!
The one thing that caught me totally by surprise was the pressure inside my ears when I was under water. I had a ruptured ear drum a number of years ago which certainly made itself known while I was in the shark cage. I guess it makes sense there would be a lot of pressure because the aquarium is huge, but I didn't think about it until we were under water.
After your dive you have the opportunity to shower and dress, then you get a cool T-shirt to commemorate the event. I would do it again in a heartbeat. What a wonderfully unique experience!