If you're into diving you should visit probably the most popular snorkelling destination on the island. The many coral rocks located here are inhabited by a rich variety of colourful fish. Among the most beautiful species to be found here are e.g. the needlefish, various turtles or the lionfish.
However, it is a very popular spot so be prepared for many other swimmers and divers. Mind the weather conditions and the safety recommendations when snorkelling. There are places to get food and other facilities located nearby. You can either get here by car (there are parking lots close to the beach) or you can also take a bus.
Wikipedia https://en.wikipedia.org/wiki/Hanauma_Bay
Official website http://www.honolulu.gov/cms-dpr-menu/site-dpr-sitearticles/1716-hanauma-bay-home.html
Address United States
Coordinates 21°16'14.874" N -157°41'48.62" E