Parking is a huge problem at Hanauma Bay and the small lot makes it extremely difficult to find parking after the morning rush. They will physically close the lot and you will not be able to enter. However, as the morning beach goers leave in the late morning, parking spots will open up, but there are no guarantees. I can't picture myself snorkeling at 6:45 a.m. either... but if you want to get parking, get there early, take TheBus to get there, park down the hill from Hanauma Bay and walk up the hill, or illegally park (as so many visitors do) on the slightly dangerous coast highway (I don't suggest or approve this -- last I was on the highway, a women's rental car was totaled trying to parallel into the shoulder, which in fact overlooks the 30 foot drop).

Good luck and there's more info on this board on Hanauma Bay as well as the parking hours and do remember that the marine reserve is closed on Tuesdays.

Aloha and Mele Kalikimaka!