Justin, see my review in the Carnival section for answers for your questions here. But to answer this question, you can definetly book a private driver tour or get a group of tour going right at the port in St Lucia. There are several tour guides there and you can negotiate a price with them. Note the more people you have the cheaper the tour is. ALSO note that if you are say 4 people, the tour guide will most likely load you onto his mini bus and then go back and try to get more people anyway. They don't like to go with only a couple of people on the bus. (This is true for all of your ports you are asking about) SO, its best if you can meet up with others to share the cost OR you ask what the price is now when the additional people get on the bus.
You can take a tour, see the Pitons, the rain forrest or plantations. There is plenty to see. Just ask the tour guide where he would take you for the quoted cost or ask to see a specific thing and you should be set.
I did not look, but I don't recall seeing boat based tours (like a catamaran sail or such) being booked at the pier. I'm sure there was, but we were not looking for it. I would not worry about not being able to find a tour to your liking and budget.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007