coreypenrose:
Watch your bouyancy - you're going to be wearing extra gear and you might pop up in the last fifteen feet if you're not careful. Also, the general rule of thumb for 7 mil suits is ten percent of your bodyweight plus ten pounds, so add on the lead. Finally, keep an eye on your air. You'll go though it quicker in the colder waters.
In July, the water will be between 14 - 16 degrees. It will be warmer than the surrounding waters due to a warm flow that encompasses the island chain. You will need a hood and boots and gloves. Dive Tutukaka have a full range of hire gear that you can try out when you go for your trip. I normally dive a 5mm, but I probably have more body fat then you do.
It's really what you are used to.
As to RikoRiko cave, you will go into it with the boat and I have dived it. Great light show as the sun shines into the cave.
It bottoms out at about 14 metres.
As the others have said, buoyancy will be an issue. Also the constricted feeling you get with full gear and the extra weight you have to carry. Take it slow when you gear up and when you are on the surface, adjust everything so you are comfortable and don't be rushed in descending by your buddy when you go down. Once you descend you can adjust and tighten up again when you get below 10m.
Because you will be wearing gloves, place all your gear, such as torch etc ( lots of great caves, nooks and crannies so hire a good torch) at the front of your BCD so you can feel for it easier. When you try on your hood and mask, set it before you go down, in fact, try it out and adjust it at the dive shop first. Not too tight because it's a bugger to change things in the water with gloves on.
Normally there is wave surge to cope with. It's not dangerous, but at the 5m stop, aim for 7-8m for safety. Stick to the walls to help keep your depth.
Don't use any seasickness tablets as they can bring on nausea. Make a lunch because the boats only supply tea, coffee, cup-a-soup and you will get very hungry, partly because of the cold. You'll be out for 6-8 hours.
make sure you bring some warm clothes and a couple of large towels. A warm hat will be a must and a good jacket.
As to the marine life?.....Wow!!!!! You'll think that you have fish coming out of your pants. Tons of pelagic life. Bring a camera. Even buy an underwater disposable if you have to.
There are feeder stations, untold colours of morays, heaps of rays and lots of nudibranchs. There may be a little murk from plankton, but I have never had a dive with viz lower than 12 metres there and I have done over 80 dives there. Even in rough weather, there are plenty of sheltered places to dive.
You'll have a great time. If you have any questions, than give me an email and I will answer them for you.
Cheers Big Ears