Catalina is beautiful, even if you dont book any shore excursions just exploring the island is fun. We actually did the discover scuba diving excursino through carnival and this was one of the coolest things ive ever experienced. It was very scary at first, trying to get used to breathing underwater, but once you do its amazing, we went 40 feet i believe, under and had great instructors that made you feel very safe. I wish there was more to see underwater, it was all fish and sea plants, but thats Catalina. Its definitely something I want to do again. I do think its possible though to go ahead and book excursions once youre at the port no need to do it so far in advance, you could probably also get a better deal not doing it through carnival.