Captain Tama's cruise is a must. It was the best activity I did when I was over there and I know many others that say the same thing. I didn't get to go diving or fishing, but that's meant to be pretty good too.
If you book anything, make sure you get it in writing or see them write it down. If you're on the island when making a booking, go and see them to do it. The whole place runs on "island time" and they don't really honour bookings etc like we're used to over here.
The scooters are a must. Great fun.
The best places to stay are all on the south side where you are inside the reef. Great snorkeling.
If you are going to rent a home and want to cook your own food, you can take things like vacuum packed meat with you. The food over there is expensive. You can get a 1L bottle of vodka for less than a packet of sizzlers.
Duty free alcohol is really cheap over there, so are cigarettes. If you want either of those with you, I'd suggest getting it over there rather than in NZ.
As far as the camera stuff goes, I got a $50 generic camera bag from Ski and Dive World in Christchurch. For a small point and shoot camera, it does the job fine and is rated down to about 5m. Allowed me to take hundreds of photos while snorkeling.