There are many more snorkeling/diving spots on Maui than this map shows, but here are some of the more popular spots: http://www.hawaiisnorkelingguide.com/maui_snorkeling.html
I personally love Honolua Bay, and know many others who enjoyed diving there more than Molokini. If you do plan on going to Molokini, take the earliest tour that you can, as that area gets crowded REAL fast in the afternoon.
I personally recommend Trilogy for any of the tours. My wife and I have been on numerous tours with them - and have two more coming up later this week - and love them. They're also the only company that can actually get you ONTO Lana'i; others can only get you just offshore. On top of that, they do a lot of good for the island, as well (Blue'aina reef cleanup campaign, etc.).
Anyway, you're bound to see a ton of wildlife no matter where you dive in Maui - especially Honu (green sea turtles). They're everywhere. Just remember that it's illegal to bother them.
As for the earlier comment about sunscreen, you really just want to make sure that you don't use any type of aerosol sunscreen, as that's the kind that will hurt the wildlife.
Great information here, last time I dived in Maui I was introduced to a number of more "local" stuff just be chatting a few people up. Some great shore diving and of course the big ones have already been mentioned.