If you pre-book the excursions, they go right on your credit card. You can still cancel once onboard and book others and get a credit, so if you see something you really want to do, go ahead and pre-book it, the tickets will be in your cabin when you arrive, or your cabin attendant will deliever them shortly after you get there. We did have one problem with an excursion that was paid for, but no tickets. I did finally get it deal with, although we did miss the excursion, but did it on our own anyway and got the amount credited to our credit card, no problem.
I have already pre-purchased the only one that I am doing through the ship at RCCL's website it was real easy (you just need your reservation number to book) and they do charge a credit card at time of booking. You can change excursions through there site or cancel upto 10 days prior to your sailings with out penality. Go ahead book away it was fun to book through RCCL's site.