I have had this happen and no, was not able to book once onboard, however all is not lost. Most of these excursions can be booked at the pier when you get off unless it is something very unusual. Once in Hawaii, we wanted to do the Atlantis submarine , but waited too long to book onboard. We went to the Atlantis dock and two people did not show up so we were able to go. What was it you were looking at?