Since no one's touched on Oahu, I'll put my 2 cents in since I live here. Oahu gets bashed because all the people are here & Honolulu & Waikiki are very urban. However, the island has a lot to offer outside of Waikiki. There are terrific wreck dives, I especially recommend the Mahi. If you dive the Mahi, the second dive is usually Makaha Caverns which has terrific lava tubes, tons of turtle cleaning stations, and I saw the biggest frogfish you can imagine there.
As for outfits, I strongly recommend Oahu Dive Center. Not the biggest outfit, but by far the friendliest, most knowlegeable & easy to work with.
There are good shore dives on the North Shore, but only in the summer, as that's where the BIG waves are at this time of year.
As for the whales, if you get in the water before they leave, you're almost guaranteed to hear their songs. That's a pretty amazing experience.
Oh, and the Big Island, Maui & Kauai are OK too. B)
Aloha