I'm afraid that you gotta wait 'til you board to book the shore excursions. My experience with the 4 Carnival cruises on which I have sailed, there will be no lines when you board. Simply go to the shore excursion desk in the main atrium, Empress Deck, ask for a shore excursion brochure, choose those that appeal to you and drop the form in the box provided. Follow this procedure and you are just about guaranteed to get the shore excursions that you desire.
We were on the Carnival Spirit this summer to Alaska. They have a shore excursion info night, 1st night I think, and you can fill out your shore excursion requests there. At this meeting, there were not very many people, and we got everything we wanted. You give your order form to the people there and they were very helpful as well.
I'm sure glad we went, because there were plenty of people on day 4 still booking excursions.
if your going to st thomas email me i have a great guy that we used for the whole day. he only charges $20 pp and is on line. this was our 2nd trip to st thomas on 1st trip we used ships tours but this beats them hands down
Carnival did some testing of the pre-cruise tour ordering this past Summer on some of its cruise ships/itinerary. To date, there have been no announcements if this feature would be implemented on any ships in the near future. Stay tuned.