Hi kellie37!
SimonV and I have both done the Sharks Deep Dive and we recommend it highly. It gives you an astonishing view of the shark reef's inhabitants, up close and personal. Many of the sharks swam right next to the cage during our dive, and we could see their eyes turn to look at us as they passed. Very cool! We also had a fish join us in the tank, which was quite fun.
The experience is even better now, as you use special helmets that allow you to breathe without snorkles (that wasn't available when we did the dive; great new-ish feature!). I'm not sure, but I suspect the helmet also relieves a bit of the pressure you feel in your ears when you're under.
Another cool aspect is, you can see guests pass by, in the walkway used by park guests for viewing the shark reef. You can also see all the way into Shark's Underwater Grill. People in the tube were taking our picture and waving at us, and we could see people waving at us in the dining room, too.
It was an incredible experience. Educational, but also a treat to do something so unique. We loved it and would do it again!