When you are on Maui, see if you can book an excursion, either through the cruise ship or private, called Trilogy. They do a catamaran sail to the island of Lanai, which also has an underwater preserve. Fantastic excursion. They give you great cinamon rols on the trip over, serve you lunch, give you a tour, lots of beach time etc. YOu can look them up online and call them. They can take a look at your schedule and see if it will fit. It was without a doubt the best excursion I did while staying in Hawaii.
My dad and sister are going to Hawaii this spring on NCL, and Trilogy is offered through the cruise line for them, so you might be in luck with Princess.
Other great excursions on Maui are the Road to Hana, and the early morning visit to see the sun rise on Haleakala. Have fun!