I go snorkeling in Maui everytime. Don't snorkel in Lahaina town. The water is murky from the harbor. Go just north to Black Rock or up to Honolua Bay. It's worth renting a car. Also south to Wailea area is amazing too. I've actually been surrounded by 20 spinner dolphins while snorkeling there!
Go to a local snorkel shop and get a map. Most give them out for free. Or go to the Maui Information Guide for a snorkel map
and choose the best spots. I also hear that the Olowalu area has a lot of sharks, so stay away from there unless you're adventurous. Aloha!