Another +1 on what Chris recommended. Generally, when my wife, daughter, and I go to Hawaii (so far Big Island, Maui, and Oahu), we prefer to explore the island on our own. Search the web and find some free snorkeling spots off the beaches. There's a ton of them. On Oahu, I'd check out Hanauma Bay, but get there really early because the parking lot fills up and they'll stop letting people in. Also get there early so you can hopefully score a spot under a tree because it does get hot there and you don't wanna get baked. The snorkeling there is ok, as long as you go out passed the break and into the bay. It'll open up and the clarity gets really good out there. As already mentioned, Shark's cove on the north shore is great as well. Lotsa fish once you get out in the deeper stuff. But I remember it gets kinda crowded and it's not much of a beach. Just a little hike down into a rocky area. But again, the snorkeling was great. I personally don't like those snorkeling boat trips because you're paying for snorkeling, which you can do for free off a bunch of spots from the shore. But that's just me. If you wanna just chill on a nice beach, head out to Lanikai/Kailua. Awesome beaches, good swimming, and alot less people/tourists.
Just my 2 cents. Have a great time and enjoy.