You really have to do this - it's pretty amazing. The gift shop has underwater cameras you can rent for like $30 - get one. All of the money supports a really great cause so you have fun and feel good about it. We swam INCHES away from multiple sea turtles - it was amazing.
This was the only mistake we made the entire trip. If you are under the age of 60, this tour is not for you. We should have just rented a car and done it ourselves. Nothing like being herded around a bunch of tourists traps to make you feel like cattle. Skip this or do it by yourself (or with friends from the ship). Seriously.
You only get 10 mins at the volcanoes but 40 mins at the nut and orchid stores. Very strange. I wanted to see volcanoes not buy a bunch of random stuff.
The tour guide was nice and very informative - again, this is geared towards a much older person.
We went to the Luau. I wasn't really sure how it was going to be - it had all the right features to be a total tourist trap - but to my surprise it was absolutely lovely. The food is TRADITIONAL so if you don't like rice, pork and the like - you will probably be unhappy. It's a luau people, you go for the experience. We seriously had the wort seats in the house and still managed to have a great time.