On Oahu, check out Waikiki aquarium and Hanauma Bay (for snorkelling). Don't waste your time and money at Sea Life Park, unless you have kids that will enjoy it, because as adults, I can pretty much guarantee that you will not. I really liked Kailua Beach-- the water was calm for swimming, the beach was beautiful (especially in the morning and evening when there aren't many ppl there), and when we went swimming, we were surrounded by green sea turtles. You can kayak there too. Just south of there, you can snorkel at Lanikai beach. The beach isn't near as nice as Kailua, but there are big coral heads right off the beach. There's no need to take an expensive boat out, and the water is really calm so you can snorkel out pretty far. You might want to ask a dive shop, though, because I know snorkelling conditions vary depending on the time of year due to ocean swells. THere may be some better places.

Enjoy yourselves. I didn't like Oahu very much. I think its way too populated. I bet Maui and Kauai are a lot better!