I had the chance to dive MOC back in October when I visited my cousin there. It was one of the more interesting dives to date. More interesting then the shark feeding dives in the Bahamas, more educational too.
You and your guest with 3 other groups (4 divers total, 1 guest each) will view a video they produced, very informative on how important sharks are to the people of Hawai'i. Then you'll quickly do a briefing. During this time, you will know what sharks they have in the tank. The tiger was released a few weeks before so I did not get to see it.
First group (2 divers) will climb up a flight of wooden (I think) stairs and suit up. The main tank at MOC does not have a permanent roof. When I was there, there was a blue plastic tarp (?) over it. You will know what I mean when you get there. They will ask you to slowly get in the water and will quickly due a buoyancy check. You will go over the tunnel, and descend in front of it.
The dive was about 30~35min (don't have my log). The guide will bring a bag full of chow for 'miss piggy', their resident eagle ray. She'd try to knock your camera out of your hands if you have a handful of shells, watch out. I was amazed at the algae build up on the wall facing aquarium visitors.
BTW, don't bother looking for shark teeth; they will give you one as a gift after the dive.
Besides your camera, bring water or a refreshment of your choice and snacks if you tend to get hungry. Have a great time....
BTW, picture is of the 2 guides at MOC and 'Miss Piggy'