I rate the Sharks Deep Dive VERY highly having experienced it last May. It is an amazing experience, even with just a snorkel (it would be even better if you are scuba-certified and can stay underwater for the duration), and the half-hour in the tank just flies past (you also feel a bit like a celebrity in the cage as everyone going through the Sharks tunnel waves to you!). The lead-up educational programme is very good, too, and the attention to detail (good changing arrangements and high-quality bath products to shower with) and extras like the special T-shirt make it well worthwhile, IMHO. Do it together with lunch at the superb Sharks Underwater Grill and you have a day to rival Discover Cove, I reckon.