Check out the aquarium. It's small but incredibly well done. You can get a week's pass, I recommend getting that because it's good value and it's worthwhile going more than once.
I wouldn't worry about locations, Maui is small enough that, to a point, everything is a short drive (no more than a couple hours). It's big enough that you don't feel on a tiny little island, and there is a rush hour now. When I first went to Maui I was maybe 12 and the population was about 20,000. It's now about 150,000 and that does not include the tourists. If you get stuck in rush hour it can be a slow go. Kihei to Lahaina takes several hours if you leave at 3. If not rush hour it's more like 30 minutes for the same trip.
I don't really remember finding any decent aquarium shops. To be honest, moving to Maui is a dream of mine, and if I ever did make it happen, I likely wouldn't have an aquarium anyhow.
Why when all you have to do is hop off a beach? I imagine a lot of others feel that way so I'm not sure there's really a huge market. I know Chin came back with some Hawaiian fish when he went (but he was on Oahu - and Honolulu/Waikiki is a huge city so there likely IS a market). But forget invertebrates - for one, you can't really import marine ornamentals into Hawaii (too much risk of introducing a non-native species - which is a bit ironic seeing as a LOT of Hawaiian fauna and flora is technically "exotic or non-native" - but that's getting off track). But the other consideration is that collection is heavily regulated and essentially not possible. You're not even supposed to take lava rock or coral rubble off the beach.
Diving spots - Molokina Island for sure. Honolua Bay is a treasure for snorkeling. Honestly though - just about any beach has good snorkeling. There's this one spot that is south of Wailea that is really worth doing as well. I can't remember the name of the bay.
I'll check at home if Linda remembers and post back. If not, I have it in my Maui guide book, I'll look it up.
Hawaiian waters are tropical but cold by tropical standards. So the reef formations will not be what you expect of south Pacific. The fish are incredible to see, fish love it there, but only the hardiest of coral species establish themselves in tiny colonies (Pocillipora mostly - some acros, some montis - I do have a book about Hawaiian corals which I recommend - I'll try to get the title for you later when I'm home). Alas, clams and clownfish hosting anemones aren't in Hawaiian waters.
Do try to see the sunrise at Haleakala crater. Leave at 3am. Sunrise is at 6:00-6:30, count on it taking at least 2 hours to get there (you will be sharing the road with countless other tourists who have been told "you simply must see the sunrise at Haleakala". It is worthy of seeing at least once. Up to you if you wish to do it again and again.
) Pack a jacket, it's cold up there in the morning.
If it's clear enough, you can see the observatory on the Big Island in the morning, then sun will reflect off the buildings and you can see them. Kind of neat.
Haleakala during the day is also worth doing. It's like being on the moon or Mars .. it's really keen.
The Needle ... Hana ... Ho'okipa ... OMG. I hope you enjoy yourself there.