Molokini is very popular on the Maui-to-do list! It is a small crescent-shaped island between South Maui and the island of Kaho'olawe, formed by volcanic activity (as are all of the Hawaiian islands). It is one of the most popular snorkel and diving spots in Hawaii. Molokini is a marine and bird sanctuary, you are not allowed to walk on the island and unauthorized landing is prohibited. Underwater visibility is anywhere between 80-180 feet.
You get there by boat from either Maalaea (10 miles) or Kihei Boat Ramp (6 miles). There are several different outfits - research them online, read reviews on Tripadvisor. I definitely recommend taking a morning cruise - it gets pretty choppy, and the choppiness just increases by afternoon.
I took our sons on a Molokini cruise last March, and they have not stopped talking about it since (and asking to go again!!). We brought our own snorkel gear (the boat did provide gear included in the price). I also brought wetsuit shirts and life jackets for them. I unfortunately got quite sea sick for the duration of the trip (your typical snorkel cruise will take about 3 1/2 hours), getting off the boat in the crater was such a relief! Consider taking something (dramamine, gravol etc.) before getting on the boat to make sure your trip is better than mine!!
view of Molokini crater with South Maui in the background