Hanauma Bay is one of the best spots for diving and snorkeling on Oahu.
Please be aware there is a $ 3 dollar entrance fee for access to the beach.
From Waikiki take the
H1 east. The H1 will lead into route 72 (Kalaniana'ole Highway).
Just follow this highway and it will take you to Hanauma Bay. There is a parking area near the entrance to the beach.
Try to go early in the day as parking space is limited and on a busy day it can be full before mid-day and you might end up parking your car somewhere alongside highway 72 (not really sure if this is allowed though!).