I was in Maui two years ago, and Oahu three years ago. I did the snorkeling bit off Molokini, and Lahaina. We were able to swim with the dolphins. Of course I did not know it was illegal to touch the turtles, so I did. It was cool.
I also snorkled off of big beach. That was pretty cool as well. I always push myself to go down and stay down as long as I can. Went over the cliff to little beach as well. I reserved myself to body surfing on that beach. The views are awesome! If you get a chance go to little beach.
Either way, when you get there you will be amazed with the dive spots. Hopefully you have an underwater camera case. Have fun!