I'd definitely take the recommendation of a poster above who suggested that you do any classroom work you can back at home (such a shame to waste time in Hawaii in a room).
I've logged over a 100 of my dives with Lahaina Divers so I'm clearly going to recommend them (I did my Divemaster course through them) but I'm sure there are other good options too.
Personally, I'm getting close friends and family certified (it's a fantastic common interest) and I'll be using LD for this. Primarily because one of the sites they go to for certification purposes (as well as for regular dives), Mala Wharf, is only 26ft-28ft deep (very safe) and has about just about every type of reef fish you could hope to see on a single dive in Maui. Plus Turtles and cool small critters. You can really have a great dive experience while doing the courses.
Also, you may want to keep this in mind: Most dives in Maui are an early start (7am check-in) so finding something you're happy with close by may be important to you if you're not that keen on super early rises (that's my reason for not using dive shops in Kihei, Waileia etc... when I'm in Ka'anapali).
Whatever you end up doing I hope you have an amazing time, I started diving over 20 years ago and I've loved every second.