I have been on this trip and it is worth every penny of the $69 the cruise line charges! You will love it if you enjoy basking in the sun, drinking margarita's and snorkeling/kayaking. I would highly recommend it!
I went on this last October and it was well worth it. It is about 70-75$ per person through Carnival. It includes Lunch at the resort and drinks on the boat on the way there. Ther ride to the Island is about 45-60 min. Were going back this October. GREAT DEAL.