Great info from the other members posting. Just to add a couple of things. The "Maui Revealed" book has got to be the best. We use the whole series (there's one for each of the major islands) when we go to Hawaii. In particular, read the section on snorkel tours to Molokini. Each of the boats have a designated mooring spot and some tours have better spots than others. If you do get a chance to do an interisland trip to the Big Island, Captain Cook's monument (Kealakekua Bay) is comparable and I dare say actually "nicer" than Molokini (here's a good reason to go back to Hawaii
). The other book that we use almost exclusively is the series called "Snorkel (
insert name of Island)", by Judy and Mel Malinowski. Their maps and directions are great. Besides that, have a great trip and take lots of pictures to share with the rest of us.
