Belize is a good place to see whale sharks...I believe March - June are great months for spotting them there. Hawaii...not so much. My good friend here on Oahu has taught scuba for 10-years, and in that time, he's encountered a whale shark only once or twice. It does happen, but it's exceedingly rare. I've been diving here pretty regularly for 2-years, and have yet to encounter one. It seems to be a more common phenomenon in Belize.
Hawaii is a marvellous spot for sea turtles, though. Green sea turtles are frequently spotted, and even the odd Hawksbill, although that's much less common. They can be very curious and swim right up to your nose...one even tried to nibble my fiance's [long] hair. I guess the little tyke thought it was sea grass! There's also a great deal of endemic Hawaiian marine life, which is stunning to look at. However, Hawaiian diving, while lovely, is very different to other warm water locations where soft corals are abundant. Hawaii has only a handful of soft corals...this is hard coral territory, and consequently, the reefs lack that '3D' structure that's so common in Caribbean destinations, for example. Topography here is provided by underwater lava formations, which are stark, stern and beautiful. In other areas, the reef is low-lying and populated with 'bunchy' hard corals like lobe, cauliflower and antler corals...low-lying so they don't get pounded by the surf here. It takes some time to appreciate, is all I'm saying.
Hawaii lacks dedicated dive resorts, unless you want to sign up for a live-aboard off the Big Island. On shore, you'd sign up for your dives through a shop or an individual guide - people do both - and spend the day diving before heading back to your lodgings. You can also shore dive extensively on Oahu and even more so on the Big Island - your only costs will be equipment rental and a guide. To see night life and so on, you'd have to have rooms in town or drive to town from your digs in a hired car. Oahu does at least have the 'attraction' of Waikiki entertainment, although I'm told that diving on the Big Island is far superior. However, your night-life options won't be as varied in Kona as they will in Waikiki.
Sorry I can't speak for Belize, but that's the HI side of things!
PS There's a Hawaii Ohana section on Scuba Board, which could give you more information. There's also a Caribbean section, which contains some information on what to expect from Belize. Good luck!
