Having been to the islands many times on land and once on a cruise, I would recommend snorkeling on the big island in Kona. You can visit most of the best sites, Volcano Nationa Park, black sand beach at Ponalua, and many water falls from Hilo. Took the catermaran snorkel booked with the ship with two grandaughters, it was outstanding. All of us given flotation devices, so us old folks and the kids had no problems. Visited a small reef with lots of fish and saw many rays at a deeper depth. As an altertive, if you have limited time in Lahaina, Maui take the Atlantis submarine. It dives to about 100 feet over a reef. We saw large numbers of spotted rays, moray eels and many colorful fish. Enjoy your trip
There are so many places to suggest it might be hard to start . If you are spending time on Oahu try Hunauma Bay. The beach and water are a protected park . You can actually see the schools of fish from the lookouts, 150 feet above the beach. Snoekelling here is right off the beach.
Kona has been suggested to you. Take a boat trip out of Kona to Cooks Bay . Again a protected area. Snorkelling here is in deeper water , but visiblility can be 100 feet.
In Kuai there are many great places, but for a day at the beach try Poi Pu Beach. Often rated top beach in Hawaii and sometimes top in US. Snorkelling here again off the beach. Many tours of Napali Coast Line (think Jurasic Park) also include a nice snorkel stop.
In Maui the number one choice would probably be Molikini. The best trip there is done by Four Winds II but other operators do a nice trip as well. There is also nice snorkelling around Black Rock off Kanapali Beach. Many other beaches have excellent walk in snorkelling. Trilogy and a couple of other operators do trips to Lanai that make some nice snorkel stops (out of Lahaina Harbor or off the beach at Kanapali).
Another alternative way to see the beauty and stay dry is a trip to the very worthwhile Maui Ocean Center.
Just a thought, Hunauma Bay is not open every day as it is protected. It is a wonderful place to snorkel, but be sure it is open the day your are there, access is limited. In Maui there are two popular snorkeling/beaches at Napili Bay and Kapalua Bay (a county beach , very popular) they are adjacent to each other. Some public parking is available.
If you visit Kauai, you can rent a car and drive to Hanalei, rent snorkel gear there (very reasonable) and drive to Tunnels Beach...it's great for beginners, intermediate or any level. Lots of fish and sea turtles all the time, and you don't have to swim out more than a few yards. We rented a home last summer near this wonderful place...you will love it! The drive alone to Hanalei town is the most beautiful in the world. The movie South Pacific was filmed there...remember Bali Hai? Make sure you have lunch there....it is laid back and fun just to walk around this wonderful little village. Just wear your bathing suits with a cover-up or a pair of shorts...that's all the clothing yhou need for the day. Check out the famous landmark church on the way to Tunnels too.