I think the only way to do this excersion is through the cruiseline. We just did it in St. Maarten and I did pre-book it through Princess on their web-site. When we went there were 18 people per boat and they used 4 that day, they now have 5. I understand this does fill up fast, so you are better off booking it ASAP especially if you want to go at a certain time, I believe we had only 2 times to choose from.
We did end up winning by a nose, excellent excersion, go for it!
Our winning photo!