snorkeling excursion available at port?

Tried booking an excursion ahead of time with several different companies and they all say that if they can't fill up the boat, they will cancel the excursion. They won't know if it's canceled until you get there. I don't want to get there only to find out it has been canceled. Has anyone been there recently and gone on an excursion you booked there at the carnival port? If so, where did it go and how much was it?