We were expecting Mazatlan to be the most boring day, but it was actually our favorite day of the whole trip. We booked the excursion for $45/person here: http://www.mazatlantours.org/kolonah...eer-island.htm
We had the time of our lives drinking all of their free alcohol, riding the banana boats, kayaking, playing volleyball, and then they cooked up some delicious tacos. It seemed a bit short as we didn't get a chance to snorkel. There were 6 in our group and we loved it.