Most of the snorkeling trips leave from Ma'alaea which looks like a long way from Molokini. If you leave from south Maui, say from Wailea, it's a lot closer, but then you have to drive a long way from Lahaina to get there. The old trade-off.
So they take off from Ma'alaea and take you out to Molokini which is a submerged volcano. Only the top of the volcano is still above water and even then only part of it. One side is totally submerged. This allows the boats to anchor in the middle or near the inside of the part remaining above water.
First you do the snorkeling for a while. Then you try the snuba. With snuba, you have a 20' air line connected to the boat. You can see much more since you're not relegated to the length of your snorkel.
If you are certified you can also scuba. Snuba does not require certification.
After an hour or so, you lift anchor and head for Turtle Town. If you see turtles, which we did, it's definitely worth it. The turtles are large (maybe 6' across). Don't miss it. Take a tour that goes to Turtle Town.