On our last cruise Oct 2008, we booked the Discover Todos Santos excursion. We were awaken at 5:30 on the morning of the excursion by a phone call cancelling the tour. It was cancelled so late that we did not have enough time to book anything else on board.
We really felt that Carnival let us down here. That said the operator had just cancelled because of low booking. I can uderstand that, but if we can not cancel a tour within the 24 hour period, the same should be true for Carnival and the Operator.