We privately chartered Mi Dushi. Since it was a private charter, we had more flexibility over the time, so instead of doing the 'afternoon snorkeling' or 'sunset sail', we did some of both. We timed it for 2 hours of sailing / snorkeling (2 snorkeling spots) and then finished with a sunset sail on the way back. Our guests had so much fun! It was quite a party. I think we had 40 people ranging from 9 years to 81! We paid around $30pp. The crew was fantastic! I was suprised that probably 75% of my guests actually snorkled. Also, its easy to think 'who cares about a rope swing?' but they couldn't get enough of it!
If you don't have enough people to justify a private charter, Jolly Pirates would be great too - we would have gone with them if we didn't have so many people participate. I exchanged a few emails with JP - they were super responsive and helpful and I was confident they would have done a wonderful job. Other than 3 people that weren't feeling well, our whole group joined us. Both JP and Mi Dushi were $30-36 per person at the time we were there, October 2009. I think you can get a discount for JP if you book online. I believe their boat is bigger so their private charter rate was out of our league for the number of people we expected.
I definitely recommend doing an excursion like that. You get to enjoy your guests without any other distractions - no phone calls, people asking you questions or other 'tasks'. We chose to treat our guests to the excursion in lieu of wedding favors. (We figured who really wants some trinkets with our name on them? Instead we gave them more great memories.)
We also looked at some catamaran style boats which were nice but more money for less time (gorgeous boats though). We contacted Pelican, DePalm and RedSail. Most of the excursions we considered included snacks and an open bar, though I think one of the cats charge extra for beer.
I'm sure it will be a high point of your weekend, no matter which boat you choose. It was the greatest feeling walking off that boat knowing everyone had a fantastic time. We were grinning ear to ear and our guests were full of laughter and energy (though surprisingly not too much booze)!
We treated our 22 guests to Red Sail's Fiesta catamaran sunset sail shortly after our civil service. It was a hit! The food was superb & the bar made some tasty drinks. And the mariachis were incredible. My family is usually pretty staid but after 40 minutes, we had the whole boat dancing and moving around the boat.
Our sunset beach ceremony was the following day and our reception was pretty laid back. I think our guests really enjoyed making the party last "a little longer.
We didn't charter a private boat. We just made sure we could fit all 22 people on the same trip. And it was kinda neat because there were 10 other people on the catamaran who "celebrated" with us. The cost per head was $50; pretty reasonable considering the food quality, drinks, and entertainment. I've been on sunset sails with DePalm and another outfit (can't remember the name) and Red Sail has been a favorite so far.