Hi I have taken two excursions in the past booked throught the Carnival.
I have gone whale watching -- highly recommend! I was not sure I wanted to do this it was fabulous - Lots of Whales with their calves. Neat experience. I enjoyed being on the catamaran. If you go try to be first on and go to upper level for best view.
Last year I went Emerald Pool tour - Not thrilled literally hundreds of people going to this small waterfall to swim in a small pool of water (swimming in it extends your life it is rumored). Trying to get back from the pool I almost lost my life trying to climb around all the people (I fell - literally flat on my face) :) Don't waste your money.
In March I will be there again and I may go tubing or stay on the ship. Have fun!