I'm going to respectfully disagree with Mark when he says you don't need guides. Although there are sites where you don't need guides for navigation, one of the biggest mistakes divers new to Hawaii make is to not hire a guide (or have a bad one). Hawaii's creatures and topography is so different than the rest of the world that unless you know what you're looking for and where to look for it, you'll miss 90% of what there is to see.
There's just nothing more annoying and frustrating to me than reading posts about Hawaii having crappy diving, everything is dead, there's nothing to see, don't waste your time, etc., etc. That's only true if you either go it alone and haven't a clue what you're looking for, or you've had crappy guides who aren't interested in pointing out the cool stuff.
Do yourself a big favor and hire a good guide for all (or most, depending on how much you're doing) of your diving on Maui. It is worth every dime it will cost you.