We did the one in Clearwater Beach. Based on the St Johns Pass website they look about the same. The kids enjoyed it quite a bit. They played musical chairs and had their faces painted like pirates. They got to run around shooting water pistols and later got rubber swords to keep and some pirate treasure. There was music and free beer and pop.
Every once in a while ds asks if we will do it again. I have no plans to do it again. Even though the kids had fun, I found it kind of expensive and I personally did not find it too interesting. I also tend to agree with you that having free unlimited alcohol available on what is essentially an activity targeted towards children is a little odd. Though certainly nothing got out of hand, there were plenty of people who were trying to drink their money's worth in beer.
We booked our cruise in the morning for an early afternoon cruise around springbreak time so it shouldn't be a problem for you to book relatively last minute.