Obviously bring a bathing suit, water shoes if you have them, lots of sunscreen, a hat and sunglasses. The sun is strong in Australia, you will burn faster there than you might anticipate. If you can afford it, go for the optional expenses like the helicopter flight and/or the underwater camera. The catamaran ride out to/from the pontoon can be bouncy, so bring something for seasickness if you feel you might need it. Be prepared to try as many of the options as are made available to you and have fun.