Does NCL offer on-line booking? If so, and you really want to do a certain excursion, go ahead and book in advance. Many people book with private operators before they cruise, especially on popular excursions like swimming with the dolphins or sting ray tours. It all depends on what you want to do. You should narrow down your choices to 2 per port (plan A and plan B) and research if these are things you can easily do on your own or if you need to take a tour. Then you'll have a better idea of if you have to book now or not. Be sure to check out the cancellation policies for any tour that you book so there are no surprises.