Hands down the best snorkeling in Maui is Honalua Bay. It's about 10 minutes up the hwy from Ka'anapali and you can just rent the snorkel gear and drive right to it.

You will not be disappointed. When you get to the beach (not a sandy spot to relax, but still worth going) Head out to the left along the rocks and you'll have a great time.

Google it.