Clients frequently ask if it is worth it to spend a day taking the ferry over to Lanai and taking a peek around. I think the answer to that would be determined by how many days you plan to be on Maui to begin with and the number of times you have been to Maui and as yet have not visited Lanai. Of course I think it is always valuable to visit as many Hawaiian Islands as you can but if it is your first visit to Maui and say you will be there a week, I think there is so much to see on Maui alone that I would probably hold off on Lanai until you come back next time. You can only fit so much in on any vacation so its better not to overdue.
Now on the other hand, if your intention is to spend at least one day doing some snorkeling or checking out dolphins when you are visiting Maui then you just might want to make that a trip, not to but along the coast of Lanai, because that will certainly fulfill your desire for a great snorkel. There is so much reef along the coast of Lanai and you will be thrilled with the sea life there likely including scattered pods of dolphins. And, if you are really adventurous, one of the speed rafts that completely circumnavigate the shoreline of Lanai will definitely fill any yearning you have ever had for viewing marine life in the wild.