East of Honolulu and southeast of Kuapa Pond on Oahu, the snorkeling at the Hanauma Bay Beach Park in the Hanauma Bay Nature Preserve Park is known for its horseshoe bay filled with reefs to explore. The bay is a old volcanic crater and contains flat algae-covered reefs. The Hanauma Bay snorkeling is protected from the large ocean waves for a gentle snorkel experience. When snorkeling off Hanauma Beach, you can expect to see millet-seed fish, raccoon fish, thread fin, butterfly fish, reef trigger fish, wrasse, convict tang, unicorn fish squid, whitetip reef sharks and occasional sea turtles.
For beginners, the inner bay adjacent to the beach is protected by the inner reef structure resulting in a calm, waist deep snorkeling experience. This inner bays tend to have poor visibility and can be quite crowded.
For advanced snorkelers on calm days, you could proceed out the central channel to the outer bay. The outer bay is still protected by a submerged reef system, but when the waves are up and the wind is blowing, the outer bay can be quite treacherous. Hence, the first outer bay is known as the Witches Brew. But, if you are lucky and come on a calm, sunny day, you will be treated to exceptional visibility -- matter of fact, the best visibility for a beach-accessed dive on Oahu. One could easily spend a couple hours snorkeling Hanauma bay while exploring both the inner bays and the outer bay.
The Hanauma Bay Beach Park is located at 7455 Kalanianaole Highway about 10 miles east of Waikiki via H-1 east (which becomes Highway 72). The #22 bus runs close by the beach. It is open dawn to dusk, but closed on Tuesdays. The parking area is on the ridge above the beach and requires a nice hike down, or you can pay for the shuttle down.
There is a nominal entrance fee and visitors must watch a short 19 minute video in the Marine Education Center before entering. Hanauma Bay Beach Park is by far the most popular and crowded snorkeling dive site on Oahu with several thousand snorkelers per day and over a million snorkelers annually. So, make sure to arrive very early for parking and for some solitude (think before 7am!). There are limited guest services down by the beach, so get what you need from the visitor center on the ridge above.
For more details on Hanauma Bay Beach snorkeling, we highly recommend the Oahu Snorkeling Guide eBook