I just finished a 7-day cruise on the Magic. I clipped in with my cycling shoes, which have an SPD cleat. Just like at the gym... the clip is on the opposite side of the pedal from the shoe cage.
Just speaking to the other observations in this thread... For the Western Caribbean cruise, there were two spin classes offered, one on each sea day. They started at 9:15 am. We signed up first thing after boarding (they allowed sign ups for both classes at the beginning of the cruise). Although both classes were full with a wait list, the instructor filled the first class by soliciting folks already in the gym, as three bikes were empty. On the second sea day, only two of us showed up, and someone else jumped in last minute when he saw how empty the class was. I think there might have been 10-12 cycles. So, even though the classes were booked, there was still space because some folks on my cruise didn't turn out for each session.