We're considering the same tour when we are in St. Maarten later this month. (Well, my husband is considering it -- I'm going to shop.) Through Celebrity, the cost is $199 per person, and I can't imagine the price would be much different through Carnival. Another option we're considering is the Bubba Bottom Fishing and Scenic Cruise tour, $89 per person. There are also private operators, yet I did not look into those. Others on this board may be of help in that regard.
When are you going? I'll be happy to report back at the end of this month.