I have found that booking online ahead of time is great because your tickets are delivered right to your cabin the first day aboard.
I also found that it was easy if you change your mind and you want to turn your unwanted excursion tickets in at the excursion desk; in which case you receive a complete refund reflected on your ships account. Of course you are taking chances if you want to book something new and popular while onboard. There could be a waiting list!
Hope this helps.