So many places to snorkel, you have to try them all! Are the Humpbacks still there, or have they started their migration? If you go to Molokini try to get on Quicksilver, it's a great trip!

My favorite is mile marker 14 south of Lahaina.
Although I saw a manta ray at Sugar beach.

15-20 minutes south of Kehei.

http://www.hawaiisnorkelingguide.com/ahihi_kinau.html

The best we saw was Kapalua north of Lahaina. Its good this time of year. In summer Honolua is supposed to be great, but I understand it has higher surf especially in winter.

Second this. Great spot. Also Mekena Landing.

Maluaka beach
There are bathrooms there. When facing the water snorkel left around the rocks. Fish and corals galore, even an arch and turtles. Go when the surf is down.