We have been to Maui a few times and have stayed in West Maui in the Napili/Kapalua area. I think Kapalua Bay and Napili Bay are excellent places to snorkel. Both bays are very calm (especially in the summer) and have some nice fish to see...easy spots for a new snorkeler.
Another spot my family enjoyed was Honolua Bay (just a few miles north of Kapalua). It has amazing coral formations and an abundance of fish with a few turtles too :). The right hand side of the bay is best. When you first start swimming out from the shore it is a bit murky, but clears up the farther out you go. It is best during the summer, but even then can get cloudy when a heavy rain has ocurred. My mother was a newbie and she used one of those styrofoam round noodles under her stomach to help her out when we were at Honolua Bay.
Snorkelingis also excellent at Honokeana Cove, right in front of the Napili Point Resort. We snorkeled there many times during our stay at Napili Shores. We were amazed at how many turtles we saw. Entering into the bay can be tricky though.
We rented our snorkel gear from Boss Frog's Dive and Surf in the Napili Plaza. Napili Village also has a snorkel shop. When you rent your gear, it pays to get the more expensive mask and snorkel. It is a world of difference for your visability and for your ability to see clearly.
Morning hours are usually the best for snorkeling (up until 12 noon) because the water was calm and the sun was bright (but not yet strong enough to burn your back if you wore some waterproof sunscreen).
