Anyone going to Hawaii should definitely spend some time in the water enjoying the beautiful fish! Hanauma Bay is the most popular place to go to snorkeling on Oahu. There are so many different kinds of fish, and the water is very calm. It is the perfect place for beginners. However, it can be very crowded, so sometimes you need to swim out quite far to be "alone."
One of my favorite places in the world is Lanikai. It is the most beautiful beach you will ever come across, and it also has GREAT snorkeling. The water is very calm, and unlike some places, the ocean floor is mostly sand. There are huge corals growing out of the sand just off shore and thousands of fish live in them. It is also very secluded.
And lastly, Waikiki beach. Most people stay in Honolulu when they go to Oahu, and although the beaches get very crowded, there is some great snorkeling off-shore. My favorite place to go (and stay) on Waikiki is the Hilton Hawaiian Village. Once you get past where the waves break, there are so many different species of fish. Most people I see out there, just don't go quite far enough out. We have seen turtles, eagle rays, and not to mention every kind of fish you could think of! The best thing about snorkeling at Waikiki is that it's right outside your hotel room!
Hope this helped!