That's a good question. I've been diving here for about 11 years now. My all time favorite spot is the Big Island - that's why I chose the Kona Aggressor II last August as my first liveaboard trip, because it was close (how often does an Aggressor work this close to home?) and I'd dove the Kona side of the Big Island a few times and always left wanting more.
I used to dive at Shark's Cove on Oahu's north shore in the summer when I was in college and I went shore diving alot, but these days I dive with the Island Divers dive club out of Hawaii Kai. We dive the shallow reefs and wrecks off of Hawaii Kai, Waikiki occasionally, and sometimes the Ka'iwi Channel. It's all boat diving and ten times more convenient - and a pretty good deal money-wise too. I'm planning on going this Wednesday - it's a bit tough to get in the water when it's only 72 or 73 degrees, but that's because I'm a wimp and I hate layering and adding to my 3mm full, throwing a 3mm shorty over the top of my fullsuit makes movement a bit tougher. Again, a stupid reason not to dive but I'm saving for the Star Dancer in Papua New Guinea - I'm less than 2 months from departure and am DYING to go!
I like the Waikiki wrecks of the Sea Tiger and YO-257 because they have more structure than the other wrecks off of Oahu. I'd like to dive Molokini Crater off Maui as well as Lanai Cathedrals one day but have yet to do that. I'd also like to do some diving off Kauai - they have good shark diving and the crossings to Ni'ihau in the summer have been successful with awesome shark sitings.
I've yet to snorkel with a humpback, it's tough out here with the restrictions of the Hawaii Humpback National Marine Sanctuary. While on the Kona Aggressor II I have snorkeled with spinner dolphins a bit, and we got some in-water time with some pilot whales - which the crew of the boat even said that that's rare. I always tell people that the Humpbacks of the Silver Bank in the Dominican Republic are the place to go to snorkel with Humpbacks, but be advised.....me as a free diver, I would like to actually swim with these animals...the silver bank charters only allow "soft water encounters" meaning you can only float on the surface....pretty weak in my opinion. If you were going to try to snorkel with Humpback's out here, it would take some patience and your own kayak - that would be the most likely way of doing it...but even then it would be tough.
Have a great time on Blackbeards, I dove off of Grand Bahama Island in June of 2001 and had a blast. My only complaint was that the Bahamas were kinda boring on land where we stayed, if I ever go back I'll do either Blackbeards or Aqua Cat so I can do nothing but dive. You're going about it the right way.
Aloha!
Eric