There are a few dive shops right along the main strip on waikiki. You can also call your hotel and the concierge should be able to give you info. There are some great dives just of the waikiki beach, right in front of the Halekoa Hotel.
As far as gear, I have always thought the most important piece of equipment to own and take with you is the regulator. There is too much at stake to use a rental in my opinion. That was the first thing I bought, so if it doesn't work at 80 feet deep, it's my fault. But if you maintain it well, you can have no worries about it working. Also, a BC is a good thing to own because then you are comfortable with the set up and feel of it and you don't have to adjust to a rental every time you dive.
Check out some great regulators at a site I found, http://www.scubaregs.com.
Have a blast on vacation. I love Oahu and it is spectacular diving. And almost forgot, the water is pretty much consistently 80 degrees so a 5mm suit will rok perfectly.