Some dive recommendations:
Being a new diver either boat operation in Lahana works - Lahaina Divers has the big boats, more divers and will group you by experience with a DM leading each dive. Extended Horizons is a smaller boat, wetter ride and only goes to Lanai. As mentioned b4 the signature dives there are the Cathedrals - you could do either as they're 60' or less.
In Kihei, I'd look to Mike Severns or B&B Scuba. Ed Robinson's tends to draw a more advanced crowd. Severns is nice b/c they're all naturalists so you learn about what you're seeing. Plan to leave at 6am though, smaller boats so they go early to beat the tradewinds coming back.
For shore dives in Kihei - there's Doug at Shaka Divers - he has the dive bus. Or Maui Dreams Dive Co. who mostly do shore diving locally. They also book Maui Diamond II out of Ma'alea - they go to sites for all levels of divers/snorkelers to enjoy. There's also the big cat out of Ma'alea - Pride of Maui. They offer a dive but it's mostly a snorkel boat so they moor inside the crater and in our case turned me/buddy loose - at the time I didn't have a lot of dives but the DM on the boat was doing a Discover Dive with some pax and I didn't want to tag along. It's a pricier dive though.
The Trilogy yachts also offer diving, snuba, snorkeling and lunch and you get to hep sail over.
Contact Steve ^^ and see if he'll take you out to Umekahame (sp) on the scooters to find the Mantas. I think he has Apollo's? and they're rec scooters - less thrust and you can ride them if you get tired of holding on. Maybe do that later in the week after you've dove there a few times first.
It really not hard to use a scooter - I did it maybe my 20th dive unescorted off Cayman - just clip everything off so nothing gets tangled in the prop.
Your air consumption goes WAY down when you don't have to kick.
The only shop I'm familiar with on Kona is Kona Diving Co. - they had(have) a small powered cat which made a nice dive platform. Good people, if you call and are talking to Kerry - she owns it.
Go snorkel with the Turtles at Kahaluu (sp) Beach Park. It's only 7' deep.Be sure not to kick one at the entry - they blend in pretty well when sleeping.