You would do well to check out the Hawaii Regional forum as well as Spearboard.com. I've been to Hawaii (the Big island) as well as Oahu several times and loved spearing there. I don't know if it is true on all islands but there is something to the effect of: It is illegal to spear on any north facing shore, national parks, wildlife refuges or within 100 feet (maybe yards) of a public beach, etc.. Basically, check out the regulations before you go anywhere.
On Oahu, I went spearing just north of Waikiki Beach with a few locals and we had a blast. It was 45-60' deep and about 250-300 yards from the shore. There was an inlet about a mile south of this location where several dive boats left from when they took divers to the Corsair wreck. I would give you specifics if I could, but I don't remember the name of the road/beach. But hey, finding a place is just as fun- especially when you have the blue waters of Hawaii surrounding you!
Good luck!