Also be aware that Hanauma Bay was (at least in the past couple of years) shutting down one or two days a week to let the bay take a break from people. Check with the state park people if they're still doing that.
You also will want to get there early, early, early or you won't find a place to park if you're renting a car. If you take The Bus out to the bay and they dump you off at the top of the hill there is a shuttle bus that willt ake you down the hill and you can rent snorkle gear at the bay.
Do yourself a favor and read the bulletin board near the pavilion before you get into the water. If the Witches Brew is churning that day take care. It is at the mouth of the bay and can suck you out into the rip current. Bye, bye Honey!