Going to snorkel, any suggestions as to whom we should book our excursions with.

I have always had great luck with Trilogy. They usually use smaller vessels than most tour operators, so you feel less like you are in a cattle car. The do lots of snorkel trips including Molokini and over to Lanai.
http://www.sailtrilogy.com/pages/index.php

If you are looking for a party atmosphere and a larger group snorkel to Molokini we like the Four Winds II, link:http://www.mauiclassiccharters.com/fourwinds.html

If you prefer a smaller group at Molokini and don't mind getting up early we have had repeated success with the Maui Prince's Kai Kanani

http://www.kaikanani.com/molokini-express.html

I have been on both Trilogy and Four Winds II. I would recommend both. Sept is a great time to go to HI. I didn't think Molokini was all that great but the boat trip was fun. Maybe think about Trilogy to somewhere else. They actually sail.

I've never been on any of the excursions, but friends of mine (and they're pretty particular) highly recommend the Trilogy full-day tour to Lanai (snorkeling, breakfast, beach picnic, trip into Lanai Town if you want, etc.)

I have heard good things about Trilogy also, but have not yet done Trilogy. I have done snorkeling trips to Lanai with Pacific Whale Foundation out of Maalea Harbor. I enjoyed it I also like snorkeling at Black Rock at the Sheraton. I am not a big fan of Molokini snorkeling because of the crowds