The snorkeling excursion is usually to the bay across from where the cruise facilities are located and is only fair snorkeling. You can snorkel right in front of the facilities, but there is very little to see as it is mostly a sand bottom. Oahu is a much better choice. The snorkeling at Hanauma Bay on Oahu is as good as it gets. I would spend the money to make sure I got there, rather than doing an excursion on Fanning Island.
Have a great cruise!